Really nice... But there's some minor detail I want to point out, when 2 long notes happen in a row, let say we press z for the 1st note, and then x for the 2nd note, then we releasing z on the 2nd note, it caused the sustain of the 2nd note to drop, the player have to make sure to release the 1st button before pressing the next one, it will be harder to do if the tempo is faster (if this mechanic is what you're looking for then you don't have to change it).
Idk if this will works, but probably you can try to store the latest keydown, so when the keyup event firing you can check if the player releasing the latest key, if yes then the sustain drop, if they releasing the other button instead (the condition is false) then the sustain isn't dropping (this way the transition between 2 or more long notes will be smoother).
Also hide the cursor during the gameplay...
The game is really great nonetheless, I'm excited for the full release...
Btw here's my gameplay of the demo...