Seconding this.

I wrote a similar tool to help my friend practice piano (being a software person who dabbles in music, seemed like a reasonable idea).

To make sure I was "barking up the right tree" (metaphorically), I also shadowed her for a few piano lessons from a professional. Boy was I wrong!!

It's not that teaching/improving technique via app would be impossible, more that the things involved were completely not what I expected. Lots of stuff having to do with curling her fingers the right way, "don't use that finger, use this other one", "You'll have an easier time if you X instead of Y" &c &c &c.

I still want to someday make a tool to help self-train at piano, but now I realize it's gonna involve a lot of computer vision, rigid body dynamics, and a whole mess of stuff.