So the screen on the Pi takes up all the pins. Thats one reason for using the Teensy (a different screen would be nice, but the Hyperpixel is the perfect form factor and just looks nice and is easy to install). Another is that using the teensy + MIDI over USB keeps things flexible if the project moves to something other than a Pi to run the DAW.
Teensy only would be nice, but it would have been much harder to write the DAW for the Teensy. using the Pi + Linux, I was able to use JUCE and the tracktion engine which simplifies a lot and does a ton of heavy lifting.
It would be nice in the future to possibly (not saying you would do this, truly excellent work btw) have forks of the DAW (with slight modifications) so that we would have alternatives for other rPi like boards. Not saying that there's anything wrong with the rPi, I have 7 of them myself, all from the very first UK release of the first board up to the most current. I just know that for many it's not the easiest board right now to find for a decent price, and it will increase in rarity due to supply still not meeting demand over a year after we started experiencing issues getting a board.
Teensy only would be nice, but it would have been much harder to write the DAW for the Teensy. using the Pi + Linux, I was able to use JUCE and the tracktion engine which simplifies a lot and does a ton of heavy lifting.