Yeah, strap holds are a good approach too. You know for sure if you let go or not.
As for "progression system", it would depend on the goal. What I mentioned was about using the grippers to become a better deadlifter. I was chasing the #3 for a while, but I noticed that a lot of the energy I spent on getting better at the grippers was taking away from my ability to be a better lifter in total, and it became one of those forest for the trees sort of moments.
When I was trying to get better for the grippers themselves, I would alternate between heavy work with timed holds and light work for max reps. I also liked doing gripper ladders with the heavy gripper.
Close for 1 rep
Switch hand, close for 1
Switch hand again, close for 2
Keep switching hands and adding reps until you can't, then rest for a little bit and start over again.
"Overcrushes" were also pretty helpful. Take a gripper that's too tough to close, close it with both hands, then hold it in the closed position for as long as you can with 1 hand. Kind of like a heavy forced negative.
Some other things I did was burnout sets. Start with your heaviest gripper, do as many reps as you can, then keep moving down on lighter grippers.
Pretty much anything that works for lifting worked for grippers.