I have always had a lot of contact with my GCs from birth, usually every day! It got to ythe point where I had no life of my own and never did anything without checking with my daughter first. To be fair, each pregnancy was difficult and there were other factors involved, too many to go into here.
However when they moved 15 miles away for my SiLs job, I resisted my daughters request to move with her.
I still visit every week or every other week and sometimes more often and strangely enough, my daughter and I are closer now than ever before.
Maybe, next time you visit your daughter, offer to look after your GC for a couple of ours, in her own home, to allow your daughter time to herself, in addition to your visits to them both.
When she's at pre-school, maybe you could share pick ups and maybe have your GC for the afternoon, as well as seeing your daughter on a separate day.
Once a week is a good starting point and I'm sure you'll both find a balance of visits that suit you both. Especially when your daughter realises that you would love to babysit.