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

Grippers are only good if you want to get better at grippers. Grip strength is so specific and if you are talking real world transferability grippers are probably the worst. IMO if you want something that actually transfers and that you can still use at a desk and that offers some variety get one of these https://www.sportgrips.com/product/grip-twister/

Then for extensors get some rubber bands.

Source: I've been doing grip strength training for over a decade, my basement is completely filled with grip training tools. So, trust me bro. Honestly though, bouldering is probably the best, can't do that at a desk though.