I've never had an injury like that, so this is kind of theoretical.
An Old School grip exercise is to lay a sheet of news paper flat on the table, and lay your hand flat on the paper. Then use your fingers to pull the paper into your hand, and squeeze it into a little ball in the palm of your hand.
You have a lot of "control" in this exercise. You can focus on wadding the paper up in your fingers, or in the "top" of your palm, or what ever you need to do to avoid messing up your scar tissue.
Reaching "out" with your fingers will get your extensors going again.
I read that the wrestler Dan Hodge loved this. Here he is at 80 years old, crushing an apple.