Good job!
Honestly I can't thank of much to say for this one, it's a simple idea that works well with the one input, and it seems you've already thought about higher difficulties and multiple songs, so you've got that covered.
One thing is that the hits don't necessarily feel like they match to the song to me, which is understandable and probably would just require more practice making levels. It might also be a good idea to give slight audio feedback on a hit / miss.
Also, and this is a very minor point, the space bar can be a bit awkward to hit multiple times quickly due to it's size (at least in my experience, maybe I'm just weird), so I would recommend changing the input to something else, maybe mouse clicks.
Congrats on finishing the game!