Ok firstly I actually think you have been rather brave posting this so well done.
Secondly to the people clutching on to their pearls going "omg think of the children" give over. Your a parent, every parent has had moments of omg my lord my kid is treading on my last nerve I need a break (and that's with the biological pull that stops you eating them alive) op doesn't have that. Just because she's a SM doesn't mean that she can avoid that trap and it doesn't mean at some point she won't have that moment with her DC. She hasn't been mean she's been honest
Being a SAHM is hard bloody work, idk if I could do it full time. I go to work to have a break feom the kids as much as I love them. Made harder by the fact if one of the kids you have to watch that there's a personality clash. That happens in non blended families too. Shocking I know to admit out loud someone call SS 🙄
I think actually that contact should be when he sees his dad, what's happening is actually your a glorified baby sitter that has no control over changing any behaviours as they have been set by his parents . You also don't have the hormonal connection that stops you getting annoyed by bad behaviour so you need to be a bit kinder to yourself. His behaviour maybe due to the fact that actually, although I'm sure your great, he might just prefer time with his dad (I mean this in a kindly way)
Personally i would go back to work part time and let DH either deal with parenting on his contact time/flex work or work out a suitable childcare. Contact time is for dads or mums. No wonder your resentful, and probably so is DSS. You have a DH problem here, esp as your reaping the behaviours you didn't sow.
Give yourself a break, I would have a nosy to see if DSS isn't neotypical. My DSD regularly spooks me with some of the stuff she does and says, I mean really but when I was told the issues I was like ah ok. Doesn't make certain conversations easier to deal with but it does help offer a explanation and I now see it as a "quirk" rather than just annoying.
Also 8 is a really annoying age. Really annoying I'm sure someone else posted about her son of 8 just getting on her last nerve too. Might be a boy thing.
You don't have to like everyone you encounter or in your own family, but you should be kind to them and enforce boundaries with DH and SDS. Remember his behaviours are not a reflection on your parenting, it's a reflection of his parents parenting.