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You're probably right, but then I think about all the variation that's been achieved with keyboard switches and I figure nobody has even bothered to try with piano keys

The Kawai MP11SE gets quite close: https://kawaius.com/technology/wooden-key-actions/

I bought a Kawai ES8, and while the sound was great, the action was unable to manage the Bach Prelude & Fugue #2 in C minor:

https://www.youtube.com/watch?v=vcC4Thug-h8

Whenever your hand goes "in", there isn't enough leverage to play the black keys, leading to complete misses. On a few grand pianos, no problem.

I ended up returning the ES8 for that reason.

Couldn't you have, like, multiple tunable rubber bands at different lengths? Giving you whatever resistance curve you want? I feel like this is something that should have a mechanical solution.