If you are doing something with music or MIDI, the Teensy boards by PJRC are an almost obvious choice. The emphasis for these boards has always been heavy towards music, with all kinds of sound creation and mixing capabilities and full support for MIDI.

So I happened to have a Teensy 3.5 lying around, which I never used so far. And it indeed seems the ideal controller for this project.

As you can see in the image it has got plenty of I/O. Unfortunately it does not have 'Touch' input like the 3.6. ( But neither has every other Teensy, so I'll have to find a solution for that.) The TeensyDuino ecosystem provides a really simple way to do USB-MIDI commands. Something like this:

usbMIDI.sendNoteOn(note, velocity, channel);  
usbMIDI.sendNoteOff(note, velocity, channel);

Yep, it could not be simpler than that, I suppose.

OK, the 3.5 is no longer in production, but I suppose this could be done using the 4.0 or 4.1 as well. As mentioned, these are also missing the dedicated 'Touch' pins. But that can be solved by using something like the FastTouch library, which actually should work on any Arduino board. And in the description of this library there is even a mention of the Teensy 4 so I assume that is covered.