Tezhar Join the party! There are a lot of others out you who feel like that. including me, and I have none of the reasons you have for feeling you have done you share of childcare.
The first thing to say is that providing wrap round childcare is not a grandmother's duty. If your children are grown-up enough to have children then they are grown up enough to sort childcare out without expecting grand parents to take the load.
You also have a range of healthcare issues that should themselves suggest to your children that a lot of childcare is beyond your capacity.
Why not think through what you think you can do that will not affect your health and will respect your need to get out from under a life of childcare. It may be that being on call for emergencies is as much as you can manage, or may be a little more. Think it out and when you are clear in your own mind, sit down with your grandchild's parents and explain to them what you can offer. Be clear with them that you have health problems and have a life of child care behind you with your own children and cannot do the same for your grandchildren.
Making it clear about the first grandchild, and your other children will have fair warning that, much as you love them and your grandchildren, your physical and mental health problems mean that you cannot cope with anymore.
We have too many threads on GN from grandparents who have become their childrens and grandchildrens slaves to the detriment of both their own health and their relationship with their children.
You are in exactly the right time to stop that happening before it starts.
Perhaps a health review, if you can get one, in these post COVID times - and some counselling could help you make decisions about childcare or not and help you see your way forward to a quieter life.