How old are you OP? Do you have or want children? Now add childcare to the mix and do you think you'll be able to cope. I have a medical condition and it certainly doesn't prevent me from pulling my weight around the house. What you've done is enabled them to disrespect you hence you've got the situation you have now.
While you plan your new life, slowly start to disengage from him and his family and put barriers up to maintain emotional distance.
Don't provide additional care for his daughter apart from what's already been agreed. She wants to stay an extra day, fine you go out for the day and leave her with her dad.
You don't tell him when you have your next week/days off, he's less likely to dump his kid on you.
His mum wants you to take her shopping - point her in the direction of online or book her a cab etc. You get my drift.
Meanwhile, look for a studio/ 1 bed flat to move into. Whose name is the tenancy in?
If you're not on it, even better as you're free to move into shared house or a studio. You don't even have to tell him you're leaving.
Go out for a night out with friends to see what life could be like if you were single.
Make sure you have your own bank account & that your wages are paid into a separate bank account. Make sure you are financially independent & separate from him.
With your job, can you transfer to another city? That would be a very good way to cut ties with him very quickly and permanently.