I know that straps exist for training arm movements where the grip often fails before the muscle being targeted reaches failure, but that sounds like not what you're aiming for
I briefly worked at a manufactory where I used unpowered, fully mechanical remote manipulator arms with squeeze grips to control the fingers. I could barely use them at first. My mom gave me a simple standard grip trainer, and after about 2 weeks of using it daily (basically using it as a fidget toy) I had significantly improved with the mechanical arms.
So, definitely don't underestimate the humble grip trainer!
