Hi OP, I have a one year old with a 6yo DSS, and have definitely found myself feeling the things listed.
I wouldn't say I 'hate' him as I do love the little pickle to bits, but I do find myself with a lot of emotions on similar subjects.
We have him 50/50 and I find days he's here are all about him, my daughter loses her Dad, the house is mayhem and it's all 'I want X, Y, Z' which of course is delivered!
Screen time is rife, things aren't tidied away...goes against how I'm raising DD.
So similar that, had I not caught myself, I could see me being you in two years.
However, I caught myself after a particularly end of my tether weekend, and reframed to realise the issue is that his presence highlights DH's poor parenting. He's allowed to do all of these things, he's just being 6, DH is supposed to be in charge but is too scared of him not having the best time ever at our house and therefore not wanting to come, that he lets him run riot.
A few stern conversations, and some decisive action on my part to just book the holidays, book days out for me and DD so she has a great time (we still do things as a family group too, but I'm conscious to book us something separately so we have space), and also realising that a two child household is just more chaotic than an only child one...which is what we become when he's here.
I still get emotional and overwhelmed, as I'm sure most Mums and Stepmums do, but it's nowhere near as bad!! When I find it building, I just ask DH to parent his two children while I go for a break somewhere.
Also, I'm sure you do everything you can to ensure your DSS is none the wiser as to your frustrations!
By all means, leave if you feel it's best, but change and improvement is 100% possible.