I have to say, this has a special place in my heart. I went straight to freeplay lol. I wrote a basic synthesizer in Unity because I envisioned a game like this, but where you could hook your own synths up into it. Check it out:
https://github.com/maross3/Unity-Development/tree/main/PianoLernen
I would love to chat about it and see if we could be a potential collab fit to do something like this :)
Great game, loved it :)