I stepped back after about 18 months of trying to be the best step mum ever. Like you OP I had this choice because DP and I do not live together.
Like you OP I got drawn into spending every weekend with my DP when he had his kids. And all of his school holiday time too.
This was, quite frankly, very convenient for him because he could not initially cope with them well on his own. He got 50-50 after a long court battle but previously he was focused on his career while his ex wife stayed at home. He did bedtimes and 'helping' but I see no evidence he had been a 50-50 husband.
Anyway, he needed my help and for what I thought was the children's sake (actually it was codependency) i did step in. I went to his house or he came to mine when he had his kids. I taught DP a lot of parenting techniques. I found him a good nanny for weekdays so he could still actually go and do his demanding job. At his home, I babysat and did half of his housework.
And guess what? Just like you OP, we then got into the phase of conflict and boundary violations and jealousy from the ex wife. After a year or so of me having a great relationship with DP's kids they suddenly went from calling me 'second mum' to shouting 'you are NOT MY MUM' if I asked them to eg pick up a towel from the floor. They said weird stuff like 'you just want to take daddy away from us.' I asked them who told them this. 'Mum says you will!' Battle lines had been drawn, with these poor children's minds locked and loaded as the weapons.
This was unfortunate but actually, and it may be in your case OP, when the ex wife started acting out I got a really good catalyst for stepping back. I was able to finally reflect on WTH I had got myself into (none of us know, do we?) and I had a good reason to tell DP why I was heading at least partly out the door.
I said look this is getting very difficult so I have re-assessed what I want from this relationship. He took it badly at first because he had been getting everything he wanted, but he realised quite quickly that I had a point. He accepted it and I am proud of him for that.
My main realisation was that DP - and I really want all kind helpful women in relationships with committed single dads to hear this - is only, and only ever going to be, my boyfriend. So I will not give like a wife.
He will always put his kids above me. He lives an hour away from me so he can be close to their school. If one of them begs him to go to their recorder recital on mum's day, our plans are toast. If mum is sick and needs childcare, he goes. If one or both of the children decides to live with him until they are 30, he will let them.
I can hang around his house all I like. But we cannot plan to live together, or even to raise a dog together. The ex wife got the progression from dating to setting up home together to having children to raise together. I will never have this.
So I detached gradually, tapering back my time with DPs kids so as not to abruptly disrupt the children. We do still see DP and his kids regularly, but not all weekend/all holiday, its not a blended scene and I am no longer his pretend cosplay wife as and when he needs. We go to his house but he does all of his housework and all of his parenting. He is getting better at it and when he stumbles I don't catch him anymore. To me, it is like visiting a relative or a friend.
In return, I put my life, energy, sanity and of course my own child above him. I have to. I value my independence.
I still deeply regret jumping into my helping role so quickly. I got nothing from it really. I actually lost money over it by overspending - DPs kids unlike mine seem to be very expensive to entertain - and I almost lost my mind.
If you feel you are over-helping OP you most likely are. If you feel you are losing out, think about who is benefiting and whether that is fair. Test how your DP may react if you make things more fair, as this will be your verdict on his character.
I hope sharing my experience helps in some way.