You have a crazy good concept here, with plenty of challenge and a fitting music/graphics aesthetics.
Your new challenge now is to make it more accessible, especially regarding the difficulty: start slow, take the user by the hand and gradually let him thrive. Quill18 said something regarding difficulty in a Ludum Dare "best practices" video that I really value: take the difficulty that you the developer find trivially easy, and make that your "hard" setting.
Also I feel like for once the pressing of key is really synchronised to the music. That is fabulous. You have two ways to let your player know about that: 1) tell him upfront that hitting the keys in time will make the game easier for them 2) let them figure out that it is the case. The second one may create an unwanted "dip" in difficulty, kinda like someone struggling in a RTS finding a good army composition and rolling through several levels with it.
I think once you have taken a lot of time to fine-tune a proper progression (with actual playtest data not just rough guesses) you'll have a fantastic game that people will want to play and could actually sell pretty well in the mobile ecosystem!
Hope this helps! See you in the next one! Take care!