Hi, i did 3 runs, here are my thoughts:
I dont think there was an exact second where it stopped being fun because i didnt really feel anything strong during the game. The whole experience feels flat, probably due to a couple of reasons:
I didnt feel any sense of danger, there isnt any actual constraint or risk to the player because the notes just keep filling up endlessly and the enemy doesnt pose a big threat.
Additionally, i think the game should feel a lot more atmospheric, and its a bit strange that there are two meters essentially representing the same thing (the notes top left bar and the sing bottom right circle).
I think the game has potential and could shape up to be very interesting, good luck.