Why are you doing all of the above and where is her father and what is he doing? Why is he dumping all of her care on your and why isn't he pulling his weight? What about her mother? Her maternal grandparents? 

I think the question you have to ask yourself is if you're okay with your relationship ending if you end up being yourself and stop doing all these things. If you're fine with that being a possibility, then go as far as what feels comfortable for you. Then it's on your SO to decide if that's okay or too far. If you're not willing to risk your relationship, then you'll want to try removing yourself one item at a time, gauge their reaction, and go from there.

This isn't just your own fault. Yes, you jumped in head first, but your SO should have held you back. Your SO was selfish and continues to be selfish. If they hate the crappy TV shows but demand you watch them, too, in order for them to feel better, then you're being used, not being loved and respected.

Finally, these are changes you'll have to make and live with for 10 years at a minimum. Think about whether disengagement will give you what you need or if ending the relationship will.

You can stop doing ALL of these things and when your partner tries to make you feel guilty, which he will, just tell him that it is not your child or your responsibility. It is his!!!!! You are there to support him, not do everything for him.

I've been there. I felt like I needed to be the new dad, but I didn't have to be and that was the mistake I made. Not saying no, not saying they are your kids and I don't want to come to the park, I don't want to go on holiday with them this time. I want to stay here and have a break from kids that aren't mine. If you can't take them on holiday on your own or do stuff with them by yourself then why the hell do I need to.

Re-Claim You.

And your DH needs to pull the finger out and step up!  A serious discussion needs to be had. It’s important that he understands that you are unhappy and things ARE going to change for the sake of your marriage and for this child. He needs to be a responsible father and SD needs to begin learning some independence. You can definitely change the way several of these things work:

1. 8 is old enough to get herself ready for school. If she refuses, take her to school in her pyjamas. Once will be enough!
2. Starting next school year, walk her partway to school and then stop at a point where you can watch her continue to school. Or look around for kids in the neighbourhood that go to her school and organise for them all to walk together. My mother did both those things from the time I was 6.
3. Begin organising things for yourself to do in the afternoons – a part time job, a hobby, anything. Discuss with your husband how HE is going to figure out childcare.
4. Does the school lunch stuff not come out of the groceries? It should.
5. When the next hobby expense arises – sorry, I’m out of money, ask daddy.
6. She can do homework herself and Daddy needs to check over in the evening – this one also helps limit the sh*t TV shows.
7. Organise time out with your friends for the next few family events.  Never apologise or explain, just don’t be available. Even bio moms need time off!!
8. Put rules on the family time – she gets to pick TV shows until 7.30 (or bedtime). After that it’s grown up time and she can go to bed or play/read a book quietly in her room.
9. Availability is at your discretion. It is a gift, not a requirement.
10. Cleaning up after her is her own job unless it’s something like laundry. Then Daddy need to do whatever is necessary.

I was a single working mom for many years and I got my boys involved very early, because I had to really. They had to tidy up after themselves, get themselves ready for school and do their own lunches from about age 8, laundry, vacuuming, some general house work from about 9/10 and take the bus to school from about age 12 after we moved house, also some cooking from early teens.  Kids need to learn life skills and are way more capable than some folks give them credit for!!

The thing with this engaging is that it's really hard to walk back all of these things once you begin doing them. The people that benefit from it are offended and feel hurt that you suddenly don't want to do these things. And of course they do because you're basically taking away something that they are benefiting from. They are going to want reasons and they are going to want to argue with you about it try to convince you to keep engaged because it helps them out. So you need to be firm in your reasons but also keep them brief. Make sure they know that this isn't a debate you were just letting them know the new If you want to be nice take all those things that you've listed and gradually pass them off one at a time. If you disengage from everything all at once it's going to leave a vacuum of care in your household and your SO is going to resent you for it. He's going to resent you for disengaging anyway but if you do it a little more gradually you may be able to avoid a blowout fight.

The best thing to do is to never engage in the first place.

I would like to stop: 

-getting my SD ready for school and walking her to the bus stop - are you the only one home?  You don't need to hold her hand and get it done, but it might take a period of time to stop doing it for her and bring her up to ability to do it herself.  Perhaps your DH can be home for that period, perhaps his job keeps him away.  The thing is to transition her to being responsible for getting herself ready.  At 6 years old she can pour cereal and dress herself.  She may need help the night before to set out clothes.  No reason she can't become self sufficient.  As for the walk, find a neighbourhood child older than her and have them walk together (even if you have to pay them $10 a week to do so!).

-having her after school - DH should be able to set this up with an afterschool babysitter.  But this assumes that you and DH are clear on your split of duties.  Hard truth time - if you work and he works, then you should have money to arrange a babysitter.  If he works, you have no kids and he supports you, in my mind it's not unreasonable for him to expect you to do some of the lifting in other areas of the household.  Home maker is not a gendered title, it's a role.  So if you are being supported (or if he was by you!) it's not unreasonable to expect that cooking, cleaning and child care are the contribution of the non working spouse. (in my opinion).

-contributing to any school lunch food (you know lunch box stuff) - I assume that comes out of the joint grocery bill?  Or are you talking about splitting finances and not contributing.  If you're talking about making lunches, DH and SD can do that together the night before until she learns how to make her own healthy lunch

-paying for hobbies - yup, that's voluntary.  Assuming you're working, the bills can be split into household vs child.  It's not unreasonable, but if you draw a hard line around "your" money, he may resent it.  Not a problem, just a warning that finances cause more rifts in marriages than most things.  And SD won't always be around.  So if he makes significantly more than you do, are you prepared in your senior years to allow him to spend freely on his hobbies (because his retirement is his) and you have less money to enjoy yours (because your money is yours).

-helping her with her homework.  Dh can do this, or hire a tutor 

-having to always attend family events. Surely my partner and SD can go by themselves sometimes. My partner is constantly pressuring me to act like we are this big happy family. - Agreed.  Sometimes you go, sometimes you don't.  But if you don't build a relationship with his family now, will you miss having that relationship once she's grown and gone?

-constant family time. My partner is offended that I dont want to hang out in the lounge with them every single night. I'd like to spend time reading or writing. Not watching shit shows on TV. I think my partner hates it too and finds it even more insufferable if I'm not there. - No arguement there.  I try to join them for shows that I don't like at least weekly.  And in return I've introduced the kids to things that I like once a week.  Watching TV is not my happy place.

-being available.  Not sure what that means.  If it means being always available, that's not an unreasonable area to pull back.  But if it means that with notice you won't ever do anything with her, I'd say that's not reasonable to sustain a relationship. Couples do favours for each other.

-any cleaning that has anything to do with her. Reasonable unless it's "in your face".  If you do all the dishes in the house except hers that's a harsh lesson for a child.  But if you're talking about cleaning "her" bathroom, reasonable!  And a good opportunity for DH to teach her how to do chores.

-trying to create a bond.  - be kind, otherwise reasonable

- I know I cant stop talking to her but I'd like to minimize any interaction I have with her. Where's the limit there? Definitely NOT reasonable.  As much as I hate the line "you knew he had kids" you did!  It's fine to pull back, but no, you don't get to be rude and pretend she doesn't exist.  Or go out of your way to not interact with her. It's not appropriate to try as hard as you can to erase a six year old from existence!  If she's being a brat, correct her so that you don't spend the next 12 years with a poorly behaved child. But if she's a typical child with good days and bad days, you need to find a way to actually enjoy her presence.  Otherwise you're going to damage her, and she doesn't deserve that.  I'm not saying you have to play lego with her.  But she is your husband's child.  Worthy of kindness, polite interaction and some small level of actual affection.  The same as you would for a neice or nephew.  You don't have to want to be around her all the time, but she shouldn't live in a home where she is discreetly shunned either.

I'm curious what the general balance of responsibilities and finances are in your home.  Theoretically, your SO should be covering more costs since he is responsible for himself and his child.  You should have a smaller financial obligation since you should be responsible for "just you".  

Now, there are lots of ways to split things.. based on occupancy.. child vs adult... relative earning ability. etc.. There is not one "right" answer for all situations.  And.. it's possible that things could be still fair to you even if you are buying the groceries that feed the three of you.. if, for example, he is paying all the mortgage/rent and utilities (or some similar arrangement where he IS paying his fair share for him and his child if you were to look at expenses as a whole.. even though you might spend money on her behalf.. he spends money on you too).  But, certainly, you shouldn't have to foot the bill for her summer camps etc...

The other issue that seems uneven is that you are doing before and after school care.  Why is that?  Do you not have a full time job?  Does his job prevent him from being available?  And... if it does... are you reaping some extra benefit from him having that job.. does it allow you to live in a better home than you might otherwise afford.. or take more vacations.. have a new car etc.. if he is able to subsidize the extras.. he may feel it is a kind of fair tradeoff if you don't work or don't work full time or a job with long hours if you can help him with his child care.  I view this kind of assistance not necessarily being "for" the child.. but "for" my SO.. and for the mutual benefit of our home.  

so, yes.. he is the one who is responsible for his child but you can help him with that responsibility if that is the best option and decision for your household.  If he has to reduce work hours.. or hire a sitter or get after school care.. will that reduce your family budget?  which is your preference?  

Is the child in therapy?  Is her dad an effective parent or is there dysfunction in your own household when it comes to the girl.  The reality is that she is likely to be a fixture in your home and life for the next 10 years. (or more!).  Helping her become a person who isn't a nightmare would be high on my priority list if I wanted to stay in the relationship.. I just don't think it's all that easy to just try to "ignore" a resident of your home for all those years.. and if you become totally disengaged and pull back from his child (and him).. it may cause problems with your relationship.  I don't mean you are obligated to keep being the drudgery maid with her and nanny.... I mean that if you want to be at a point to ignore her and avoid her 100%... it will be hard to hide that for that length of time.

You don't want to attend all his family events.  I mean.. I kind of get that desire.  I'm an introvert and don't love those kinds of things.  ARe they frequent?  I mean, I understand his wish to have his SO by his side.. because to him you are his family.. and he wants you at his family events.. but there should be room for compromise if they are too frequent.

I agree with everything said above, and would just add that it might make sense to find activities outside your house for yourself for a period of adjustment.  For example, take a yoga class or go running in the mornings, so your DH is forced to handle the mornings.  In the afternoon, also find a way to be unavailable and out of the house.  In time, your DH will have theoretically created a new routine and be less dependent on you to parent for him, then you can be home again more often without so much pessure to do all the parenting?

I get the frustration.  I was the full time custodial StepDad.  However, I joined the party when my DW and I married the week before SS - 28 turned 2yo.

Now for some observations. 

Getting ready for school - An 8yo does not need anyone to help them ready for school.  So... stop helping. An hyo can walk themselves to the bus stop.  Other kids will be there as will invariably a few parents.  There is no need to walk her to and from the bus stop.

After school oversight -An 8yo is on the low edge of the home alone latch key kid range.  I find 12 to be more appropriate. However, 8 is way beyond old enough to not need direct adult supervision for every activity after they get home from school.

Packing school lunch - She is old enough to pack her own lunch. If she does not do it, going hungry for a school day won't hurt her one bit.

Financing hobbies - Her  BioParents can pay for her hobbies.  I would not participate in paying or the discussion of how it will be paid for.

Homework - She is also old enough to sit down and do her homework.  Her BioParent can deal with confirmation that HW is complete, etc....

Attending family events - I never had an issue attending events with my DW and her family.  We never participated in family events with the shallow and polluted end of my Skid's gene pool.  He did SpermClan events when he was on SpermLand visitation. I am not sure why anyone would participate in events and activities with the blended family opposition.  The Skid can do that while htey are with that part of their blended family equation.

As for the spousal pressure to act like one big happy family.... my XW played this bullshit game. I did not play it.  We would fight like cats and dogs on our way to one of her family events then as soon as we parked and got out of out car she would put on the big happy smile, take my arm, and try to sell the bullshit that our relationship wasn't crashing and burning.  My XBIL would hand me a beer, pat me on the shoulder, and tell me he understood. I never had to say a word.  He knew his batshit crazy sister and understood the hell I was living.

Availability - If you are going to sustain your marriage, availability is not something  you can eliminate.  Like her or not, SD-8 is your mate's child and the odds of a successful marriage is highly dependent on you figuring out how to engage with the kid effectively though within your own chosen boundaries.  

Kid related cleaning - An 8yo can clean up after themselves as well as do some key chores.  This is definitely a hill I would die on.  She does chores starting immediacy and each year she does increasingly more chores in complexity and volume.

Creating a bond - IMHO you owe your mate an effort to create some reasonable bond with their kid.  How important is your marriage to you?  Choose wisely on how you move forward on this topic.

  Limit to communicating with the Skid - This is really up to your mate.  How  you establish the boundaries for this can have notable impact on the durability of your marriage.  Another area that will necessitate that you choose wisely.

Like you, I am a non breeding SParent.  Though I entered the blended family fray when my Skid was much younger than your's was when you jumped in.  I met my DW when her son was 15mos old.  We married the week before SS-28 turned 2yo.  I raised him as my own and did not have the challenges and issues that you have had to deal with.

I would advise that  you not be harsh at all.  Be firm, direct, and assertive... but not harsh. As much as this Skid may be your nightmare, she is still your mate's child.

Good luck.

Im 7 years in this and SDs were 15 and 8 when I started.

I had to disengage pretty much right away from the 15 year old, she was and still is a difficult, mean, dirty person.

Just recently I have had to disengage from the SD15. I guess its the age! I really used to like her, and still do, in a limited sense.

So - reading your post and comments- 

1. Keep finances separate if you can. This will help your husband in his journey of financial health and good habits.

2. Disenage slowly. Dont discuss, strategise. Its for the "good of the child!" If you come from a place of "I want to get away" it will be met with loads of resistance. If you come from a place of wanting to help create a normal happy healthy child, then it will be better recieved, more agreeable and better for you.

3. Do not pay for hobbies. I made that mistake for 7 years, and got my head chopped off, just recently. Do not pay for sundries. This is your retirement money, your fun money. When I started spending money on myself that I would spend on SD15 backstabber/munchkin, I really started feeling so much better about life. Less resentful. It made our relationship better. Shes an ok kid, but she likes going to her mother Toxic Troll, and "activates" her, and then there are threats, and I get blamed.

SO, disengagment. I do not do for kiddo, or buy for kiddo. I let husband cook. And direct her. No more pickups/drop offs.

But in my situation, we had an "incident", and I was told to back off and "leave her alone", so it was a spoken deal about my disengagement.