What on earth were you thinking of when you took on the baby for full time care five days a week? It’s far too much for anyone and I don’t know a single grandparent who has been willing, or able to do this as they don’t have the energy. Most of those are a couple of decades younger than you and don’t have a husband with what will be increasing health needs. They also have a life of their own and want to do things without being hindered by a baby.
You have been very generous with your time with your other grandchildren, but it is now time for you to step back. Even a full day with a little one is likely to be too much given your age and circumstances. It’s challenging for people even younger than you.
I am sorry to say that your daughter, at the least has not thought this through and at the worst is selfish and self centred and doesn’t care one jot for you, her father or her baby. It’s more likely to be somewhere in the middle.
You need to clearly advise your daughter that you will no longer be able to look after the baby as the demands are too great for you, but you currently can continue with after school care for the older two. You don’t need to go into any lengthy diatribe about the whys and why nots beyond its too much for you given your age and your husbands health which will decline further.
Why should you contribute to childcare costs unless you have a large disposable income and if you do that with your daughter, you should also offer the same to the rest of your children in the interests of fairness. If you can afford to pay out a few thousand a month go ahead, but you also need to consider if you are straying into the territory of deprivation of assets and any tax implications.
It is up to your daughter and her partner/husband to sort out alternative childcare as fully grown adults and the parents of the little one. If anything happened to you, they would have to find alternative provision.