Perhaps you could start by not allowing SS into your home. Your home should be your sanctuary. Could you request that DH meet his son some place else? This assumes that you and SO share the home. If it is SO's home, then you need to find someplace that is your sanctuary and go there each and every time your SO's son enters the home. Is there a friend you could arrange to visit? A scenic spot you could drive to and park and read or listen t music?

No adults in your home without your permission. It's up to you whether you care if SS visits when you are not at home.

In my case I told DH that SS32 was never to be in our home even when I am out of town.

There is no one set way to disengage- we all have to approach it to keep our sanity and self-esteem as best we can. What it looks like in real life is what you want it to be- for yourself. The first rule of disengagement is to put yourself first; to lots of women who have been brought up to think of others, this can seem the most difficult part. No one has the right to make you feel sick and weepy just by a hint of their presence, and the word toxic is accurate. You wouldn't want to put yourself near radio-active materials, eat poisoned food, or breathe in toxic fumes. Everything within you is saying keep away from this person, and that is what you must do.
DH bending over backwards isn't right either,and why I and so many others came to this site. DH is not poisoned by his offspring- he is immune to the danger, so no good trying to make him feel what you feel.
Removing yourself from his presence does not mean you have to leave when he comes over- you are entitled to feel safe and comfortable in your own home- but many step-parents choose to find something else to do. Work on that aspect. Be prepared with something you enjoy- reading, craft stuff, baking etc.Rediscover the hobby you wanted but never had time to do.
Make a SS free room somewhere and retreat there whenever you need to, not even when he visits but just somewhere to call your own.
My skids have rarely visited my home since I disengaged. I don't buy into the circus and I think we have all moved on. Maybe when SS realises there is no game to play he will stop visiting too.
However you decide to manage your disengagement I hope you find some peace. You have a right to have it. Keep putting yourself first and you will get there.

In the case you describe I believe that disengagement would look like.... this prick never entering your home for any reason.... ever... whether you are there or not.

If DH insists on maintaining a relationship with someone this toxic he can do it somewhere else other than your home.

IMHO of course.

Why don't you leave when he is there?

Tel DH no, he can have a relationship with his child outside of your home. If/when he complains, I'd ask DH why he feels it's okay to be disrespected in your own home? Why you should feel unsafe and unwelcome in your own home?

Wash and repeat.

“DH told me that keeping SS20 away, "wouldn't be the answer." He knows how much it hurts me but blames me for letting it get to me”

Ask DH how he knows that. Has he tried keeping SS away? Has he tried catering to you and your feelings?
Perhaps he’s right but you can never know unless you try.

So, no SS in the house for 3 months. Go meet him with DH for lunch. Be as nice as you can possibly be, really suck it up and try to engage with SS. Be attentive and interested.

If it works and SS talks to you and looks at you great, maybe he can come to the house to visit again.
If he’s the same as he’s always been, you tried. You took the high road. You were the bigger person, it’s SS who is the problem and unless he can treat you with respect he’s not welcome in the house.
DH can keep seeing him on his own.

After telling SS that he had to move out. He stated that he would never set foot in our house again. Yay! SD's jumped on that band wagon. Yay!
So it has been almost a year and I have not spoken to or seen any of them. Yay!
My life is peaceful and I am saving lots of money on cards and gifts for them and their children.
It's sad but OK.
DH talks to them on his cell phone I am guessing when he is at work. He texts them. He sees them only when a gift is due because they call him or text him and invite him to meet them out so he meets them with gift in hand and although they invite him, he pays.
DH knows what they are all about but they are still his children and he loves them.
They try their best to try and hurt me or cause me pain by making sure that if there is a party that they invite DH and specifically state that I am not invited. Trust me, they are doing me a favor! I wouldn't go anyway but would have sent a gift. Now I don't have to buy a gift.
What I did learn through all of this is to protect myself and my children. I am determined that if I pass nothing is to go to his children.
If you leave your home you are probably thinking #1 why should I this is my home and #2 if you leave you will have no idea what he is getting into while you are gone.
Just pick a room and plant yourself there if you want. If not make alternate plans or suggest that DH meet SS out somewhere.
My SS had no respect for me, DH or our home. I told DH that none of them have to like me but they should respect that I am their father's wife. And since there is no respect... I don't need them in my life.
Things are so much better now. There are blips and it is hard for DH not to talk about them and it is hard for me not to ask about them but we had a bit of an argument a few weeks back and I told DH that it was best that he didn't bring SS up to me again because those conversations never go well. So far that has worked and if he does bring him up I don't respond, I leave the room or whatever. I do not engage in any conversation that has anything to do with SS.

I had to disengage from my SD14, and she lives with us fulltime. When I'm not dawdling at work, running errands, or visiting my family and friends, I spend a lot of time in my room watching Netflix.

I'm not allowed to be "mean" to her, because her BM desperately wants custody (CS) back, and I can't be "abusive". But the day she turns 18, all bets are off. I'm taking my house back.

I have excellent verbal skills, and can be quite devious when I want to be. I intend to make my house very uncomfortable for her. Sarcasm is a free service I provide. She most likely will not want to come around. And if she loses her temper and says something to me, even better, I can tell her to GTFO.

How are your verbal skills? What would set him off? If you have the stomach for it, you might want to consider the satisfaction of making him uncomfortable and driving him out. Maybe try addressing him directly and pressuring him to respond, asking him why the long face and then be amused and condescending at his petty problems, or follow them when they leave the room, or chide him for being rude, or serve him coffee and give him a chance to be ingracious, or serve lunch and force them to come to the table.

In short you are not disengaged.... you still have to reach the stage of I give a eff...... this is not easy for people like who who cares,

Now this is what I did to get to that stage, My house my rules, I will not leave the house to make a brat happy, I'm sorry ...
In my house I'm the Queen bitch, if you don't like it do not come over, I gather SS is not living there and visits... make it uncomfortable for him to come over, hell I even made it uncomfortable fir Aergia who lived there while a minor.

If SS ignores you, ignore him right back, what your husband do with his son is their problem, if you need your husbands help you simply say, Hon I need your help can you please come and take out the trash/cut the meat... you simply pretend that SS is not there and you go on with your life like you normally would.

If you and DH are busy talking and SS chirps in, look him in the eye and say, aren't you to old for this rude shit, I'm busy talking to my husband (never say your father) then simply go on with what you and DH have been talking about. If DH then turns away from you and engage with his son, say nothing tell them to take it else where and ignore DH, you do not even cook dinner if you have not started. You say nothing to DH in front of the snot. Ignore DH for a day or 2 and then simply tell him cool and calm if he ever disrespects you in front of people again there will be consequences.

You stop fighting, you stop nagging, you simply go into ignore mode, you have to teach your husband now , he's the problem.. the kid is an adult and nothing to you

As fairyo said, there is no right nor wrong way to disengage - it is different for everyone. Yes, your home should be your sanctuary, but in practise, it is not always.

I feel I have reached as good a compromise as I can regarding my own SD21 and SDstb23. Most of the time, DH goes to meet them in a cafe or a pub and buys them coffee or lunch. About twice a year, I cook a meal for them at our house - they are usually polite and civil on these occasions - if they were not, I would not continue to do this.

It is abusive to ignore someone or walk out of the room when they walk in. If this is the case and your DH is not willing to address it, I don't see why you should tolerate any more of these visits.