As mentioned before, a Leonardo / Micro / Pro Micro can generate button presses with the Mouse and Keyboard library. Then you don't need the JoyToKey anymore.
There is something I'm missing: tactile feedback.
Can it be made mechanical with a adjustable force and with switches or buttons ?
The user should feel and/or hear when a button is accepted.
Should the feet stay in the same position or is it possible to move the front of a foot to left and right ?
What if there is a (rubber) shape to keep the heel in that position. I assume that it will be hard to look down a lot to check where the buttons are.
When a FSR is used, then I would add hysteresis, since a foot could shake a little. For example accept that it is pressed when > 700 and accept that it is released when it is < 400. Perhaps a smoothing filter is needed.
Perhaps the code can check for how fast the pressure is increased instead of fixed values.
Should it be possible to keep a button pressed ? Then pressing and releasing should be processed.
What about adding a button (for the feet) that selects certain settings ? For example a setting for scrolling through a webpage, and an other setting for other tasks.
How does the user move the mouse button ?
Why not make three different ones and let the user decide which one is the best ?