I can talk a good talk, but it might be worth adding that, if not obvious from my post, you and your daughter will probably find the most difficult thing will be other people.
Pressing you to press him into conforming.
Excellent opportunity to use a ton of cliches - it's a Marathon, not a sprint, it takes a village (of people who know what the f they're talking about), make sure you're wearing your own oxygen mask before worrying about anyone else.
I'm reminded of this because it's midday Wednesday here, and I'm done with this weekend.
It can be exhausting. It's not the parenting that's difficult, it's the being a carer.
I see glimpses of parenting when the stars align and both children are where they're supposed to be, and I absolutely love it. Being a carer, not so much.
You daughter/ her family, are very lucky to have you, but it can be hard when you have different experiences of the same child.
My ILs are terrific, and not big on confrontation
So if I say "Bob will need to excuse himself from the family lunch when he's over it" they'll just say "of course ?! Of course!!". Where's others in the family often try sending me links to aromatherapy, tough love, all sorts.
That's fine if it's an email (that you can ignore or block) but when you're actually in involved in the day to day parenting, someone has to have the last word, and presumably that will be your daughter. So, absolutely communicate as much as possible, keep listening to each other, and make sure you can both tag in and out when your need to.
Diarise serious breaks for all of you. At different levels. Maybe a weekend away one or twice a year with your other half / her and her other half as relevant.
But also monthly, an afternoon simply to have a hot bath and an afternoon sleep.
That kind of thing. Not in a book, not often discussed, because so many of us cannot do this, but you're starting at the beginning, and offering a different level of support. If I was at the beginning (and had realised what was happening), these are things I wish I'd been told.
So much easier to enjoy the sweet moments when you're not permanently burnt out.