Make it easy to understand timing/notes mistakes
It is not easy to know why you did not get a note correctly. Since there can be 3 options (too early, too late, wrong note) can the app be changes so misses have different color according to the type of miss? This can also be applied while playing. Instead of writing too early/late just show it by color (or text with the corresponding color) so the reason will be clearer and learning faster.