This is a really cool game! At first I was confused, I especially wished that we could have more than one of each note on the screen at a time. But then I realized the power of splitters and corner notes and how you could route some of the notes through different instruments and loop back around to hit notes, and the complexity ballooned from there. This is SUPER cool!
The only recommendation I think I would have because expanding this, is putting some kind of in game tutorial that teaches you the use of each tool. It's quite a bit of work but for something this neat that isn't straightforward I think it would help a lot. Awesome job!