Nice game, by the way, you may be knowing it but for some reason, during the gameplay and when maximized, the screen was not completely visible, I don't know why, did you set Project Settings >> Stretch Mode >> Window under Display Section >> Stretch Mode >> 2D, if not, setting it to 2D would be really good if the game can't fit in multiple sizes, because it just stretches the to the display window somehow, or you can also set the resizable to False in the same settings field/group, then, the window can't be resized.

For the gameplay, one recommendation from me, I think coloring the different musical notes in various different colors would be nice! :)