Looking at it, the songs all have the same keys. All I change is the speed at which the keys go from top to bottom (basically, making the only "difficulty" based on reaction time).
Honestly, one thing I can change is how precise you need to be to get a good score. Difficulty definitely needs more fine-tuning, so, I will make an update in a few weeks with a bunch of corrections. Thanks so much for your time and your constructive criticism!