Hi user
Doing the same sort of thing expecting a different result is the definition of insanity according to Albert Einstein.
Your DP has been primed and conditioned by his parents to be the ways he is towards them. His own fear, obligation and guilt as well as his inertia when it comes to his parents hurts him as well as you.
He needs to speak with a BACP registered counsellor and one at that who has no familial bias about keeping families together despite the presence of mistreatment. Reading "Toxic Parents" by Susan Forward will also help him. He is going to have to let go here of any and all hopes that they will change; this is who they are and they are not for changing. I would also suggest you read and/or post on the "well we took you to Stately Homes" thread on these Relationships pages.
You cannot change them but you can both change how you react to them. Your boundaries for them are far too low and need urgent revising upwards. Do start being far less available emotionally and physically to them as a couple.
Re your further comments in quote marks that I have separated out:-
"I think I am hoping if I can change my feelings or pretend everything is okay then it will be, as I do want DP and my children to have their parents and grandparents involved".
May I ask how old you both are?. Doing this will just set you and any children you have by him up into having more emotional pain. Please do not do this, it will backfire on you. Your own feelings are just as as valid here so stop with the pretending that all is ok when it clearly is not. Would you have tolerated this from a friend, no you would not and your inlaws are no different. You forget here that if your inlaws are too toxic or difficult for YOU to deal with, its the SAME deal for your children as well.
Look at how your man now is around them, your children will behave similarly if at all exposed to his parents. Why would you at all want your kids to have a relationship with them anyway?. Your children would also be scapegoated. They are NOT good people to be at all around, they are not emotionally healthy enough. If they behave like this now, they will be the self same towards you as grandparent figures.
"I wish I was strong enough to say no to the constant asks for favours"
How do you behave at work; can you say no to people there?.
The two of you are being used so consider saying something along the lines of, "sorry we cannot do that, this does not work for us" and repeat where necessary.
Why are you wanting their approval (which they will never give either of you) or try to people please; these people are impossible to please and besides which being a people pleaser is not who you want to be at all. Consider also blocking their number from your phones.
Your man is also stuck in the fear, obligation and guilt re his parents and that absolutely needs addressing now and particularly before marriage and children.
"but think I feel like if I do more for them maybe they will do more for DP but it seems to be the opposite"
That's because it is. Again stop doing stuff for his parents. They see your kindness as a weakness to exploit and they will continue to exploit you both if you allow them to carry on treating you both as they have done to date.