I would suggest looking towards equipment for "overcoming isometrics", which at its baseline is literally just self resistance and can be expanded with a few mechanisms to add some springiness and help your nervous system reach full activation, since it tends to shut off if it feels a truly immovable object. The hand gripper is just a very specific device of that type. There are also systems like the Iso Trainer or Bullworker that allow you to address more of the body and from more angles. This method of training emphasizes strength in the tendons, since there's almost no motion, and it doesn't have a simple numerical progression like weights, so it's mostly discarded by people seeking "big muscles" and "big lifts". But it has a lot of substance as a general strength training method.

An overview of some equipment in this category:

[0] https://youtu.be/komva2gCaWM?si=lPwMN2UalEqtz7l1