I’m both a Grandma and the daughter of parents who are a lot like yours @StrawberryShortcake.
As I was reading your OP I was thinking ‘yep, yes, yes, yep’.
Unless you are the child of manipulative or disinterested parents I think it’s difficult to understand how it feels to be on the receiving end of it yourself. Summer bank holidays used to be the worst for me, I could hear happy families enjoying their time together and I never had that. If you don’t have a harmonious family you see happy families everywhere.
The ‘but we paid for x, y, z so we expect you to...’ absolutely resonates with me.
I’ve moved on from the way you feel now, many of my suggestions reiterate the excellent advice you’ve been given.
It’s perfectly reasonable to expect that your parents would say kind things to you. Unfortunately, those aren’t the parents you’ve got. Find a way to accept that, it will help.
Now that I’ve paid for my own children through University I do not consider that they should demonstrate eternal gratitude to me for it, paying for your children’s education is just parenting.
You are already doing this but distance yourself from them, look after yourself. When you do have to see them try not to see them at their house, in my experience they will feel able to be at their worst there.
Stop yourself from hoping and giving, you can’t change them, you can only change your reaction to them. Decide on your boundaries, make them and keep them.
Their criticisms of your parenting are a nonsense, take all of the proper advice on board but you know your own son. They’ve had their chance at having small children, this is your son, you know if he is tired and needs to be in bed.
Choose your boundaries and stick to them, for your own sanity. They are looking after themselves. You have to look after you.
It sounds as though you, DH and DS make a lovely family. Enjoy what you have and focus on that. 