> I’ll probably replace this with a bit of proto board with a couple of similar switches on it, but it might be nice to consider some kind of piezo sensor to give the instrument some expression

Guitarfreaks (the original guitar game) uses an optical encoder for this, as it's meant for arcade use and sees a ton of abuse. They basically last forever. They also use custom sanwa buttons with omron switches. A third party company (gamo2, gitaller) makes a really high quality home version.

Fun follow up might be swapping the crappy rubber domes with mechanical switches, doesn't need to be anything fancy, the gitaller uses gaterons.