I guess you didn’t have time to playtest this because the difficulty is practically unplayable.
One thing I’d recommend is replacing the “Arrow keys/WASD to select a note, Space to hit the note” with “Arrow keys/WASD to hit the note” which is a much more intuitive control scheme.
I’m looking forward to see how this game develops!
I’m also hosting a monthly jam for continuing work on jam games (among other projects), if you’re interested: Continue Your Game