Don't offer any advice on pregnancy and baby. Every woman likes to do her own thing. Unless she explicitly asks for advice.
Once baby arrives don't try to take it away from mum. Ask if she wants you to hold her. I really apreciated people doing practical things, but everyones idea of help is "I'll hold a baby". In most cases that is really not what mum wants unless she asks.
Your son is adult. You can start treating hime more lile friend than a child. That also means you don't have to unconditionally love all shit he does. Tell him you love him but xyz is irritating.
Don't center your life around them. Do your thing, you will all need some time appart.
Ask about solid financial structure and check monthly if they are on track. Explain to them that long term plan is very important for you before they move in.
Be very specific about guest rules once child arrives before it arrives.
Offer small (not baby related) delights. Cook their favourite meal, buy them drink they like etc.
Don't vorry about them staying forever. No DIL wants that.
Don't side with your DS if he fights with DIL. Stay out of it. If he is unreasonable, tell him that in private. If she is let it be as he picked her (unless there is obvious abuse going on).
Try to relate to DIL as a woman. Talk about ridicilous maternity pay, abnormal childcare cost or fashion or any other interest she got.
Tell DIL you understand it isn't easy for her.
Remember it is your house but respect guests.
You sound quite reasonable, I'm sure it will work out.