Hi there! I can't speak for everything since I didn't do any of the coding, but maybe it has something to do with my partner being in Spain? Perhaps the key placement affected that. I however have only played it with an xbox controller which feels fine. I'm assuming arrow keys work and therefore Z would be fine for that.
As for the gameplay, I think that's sort of the nature of the game. If you played just shapes and beats, you can kind of just do that. Personally I think constant forward movement in this game would be way too much with the eventually really dense bullet patterns.
As for the shapes, I had certain ideas and colors in my mind for what I associate with each instrument. The end result wasn't exactly what I had in mind initially but whatevs. I think if we were to use waves, the sounds would have to match them but there are real instruments so...their wave patterns would be a pain in the butt xD
I appreciate reading this comment and all the feedback you gave!