That's a really clever mechanic. I would just make the notes change automatically, so that the player just needs to press one button (because there is no real need to do otherwise now), the melody could also change depending on the enemy you are fighting. I would also think of different things that the note lengths would do, because I found myself just using quarter and eighth notes. Maybe playing specific notes in sequence could trigger some kind of combo.
From technical aspects, in the tutorial, you start playing notes right after the dialog. So I messed it up a lot, because I wanted to skip the dialog and then start playing notes, but I kept playing an eight note at the start and failing. Also I would use a more pleasant sound for the notes, because it got really annoying for me after just a couple minutes of playing