Questions and styles
I've tried being an all in over doin it step parent but felt I cared too much, maybe more than the parents. Not enough energy for it anymore. I tried being the "encouraging adult" or "friend" which is a little better and Im trying to move to this role.
I began disengaging when SS was here and realized I was very involved and became resentful because I feel DH is not involved enough...
He is a great father in many ways but in some ways I felt he was lacking and like I needed to pick up slack (is this my perception and expectations of what a parent "should" do and be and do I have this right as it is not my child but affecting my house?)
I discussed it with DH and he said he didn't expect it of me so I stopped but I realized when I didn't step in I was just angry DH wasn't stepping in so I started stepping in again (ugh)
I have many examples but even little things like constantly telling SS not to leave every light on in the house or reminding SS to bring things back to BM so we don't have to drop it off later mid week again etc.
Resentment still cropped up so I started busying myself during visitations or hiding in my room. Not ideal. This caused DH to want me to spend time with them (when I didnt want to) and me to be resentful of having to feel like I need to hide in my own house on my only free days off..
I'm wondering if it is fair and/or healthy to have this type of seperate relationship...DH is taking him out to do things and spending little to no time at the house. I said I'd be willing to spend time if there is something to put focus on (like bowling or a movie at the theater)
Does disengaging work for your relationship long term or is this going to make things worse?
Success stories welcome. Lessons learned encouraged 
I think that a positive way
I think that a positive way to deal with this is to engage in activities that you enjoy away from the home for a notable portion of visitation time. Maybe participate in a meal at a favorite restaurant with DH and the SKid each visitation but fill much of the Skid time with other stuff.
We were the CP household in our blended family adventure for the 16+ years that we lived under the CO. SS-28 had long distance visitaiton with the SpermClan (5wks summer, 1wk winter, 1wk spring) and we learned to enjoy our aduts only Skid free time when he was on visitation.
See about finding a related balance that will work your Skid visitation schedule.
Good luck.
To me, the examples of his
To me, the examples of his parenting that you mention are little things: leaving the lights on, forgetting his stuff, etc. As long as DH is the one making sure SS gets his stuff and not you, then let it go.
Sounds like you are the type of person who likes things "just so" and that makes having kids in the home that you can't control harder. For example, many on here get furiously angry about messy skid rooms - I used to just shut the door, didn't care.
If you can get past caring about little things, then it can work. If there are worse things going on (you left to clean everything, rude behavior from skids, etc) then disengagement is harder.