I am a grandparent carer and have had two grandsons since birth. It is okay at 40 but now I am 59 and it is hard work and they are far from babies op. I also have two other gs's (same mum) but have been hands off with regard to childcare as I feel taken advantage of. I find the more I do for her the more she disrespects me.
Your DIL sounds like she is treating you like some servant, whilst elevating her own dm. You sound a lovely caring grandparent and believe me, you have done the right thing. How dare she dictate that you should give up a job you love. Your DH also sounds like mine, it is very frustrating as I often feel by myself when he effectively opts out.
But my dd2 had a baby girl last year. She was a huge support to us with the boys and although I am a bit childcare resistant now, I am going to have her one day a week, and her MIL another day.
I had her a full day yesterday and DH helped out but swerves nappies and feeds. I feel happy about my decision but I have told dd2 that she has to remember that people do get ill (as you get older) and need breaks. For that reason she has booked nursery 2 days as well.
You sound kind and caring and your DIL does not respect your kindness. What a horrible DIL you have - MIL's take a beating on here sometimes. Do stick to your guns. It would have ended in disaster any way.
If they have financial difficulties, why can't her mother with the high flying bank job help out financially if she cannot assist with childcare? If she is so great, why does it all fall to you?
I hope matters work out for you op. You deserve better than this. Your son I am afraid to say, sounds a little bit like your DH who has left you in the firing line. Glad he has decided to step up and speak to your son.