SynRec's RHYTHM GAME MAKER plugin is currently doing everything I need... Except that I'm having some weird conflicts with another plugin that somehow prevents me from changing the font and placement of the game score. I really like the way you've set up the score UI stuff on this, might have to pick it up once you get it updated a bit more.
I'd also like to figure out how to do something like having successfully timed presses fill up a gauge for a special ability like a score multiplier or something that helps you "survive" the challenge a bit longer... Sort of like the "star power" from the Guitar Hero games.
Your game looks great! Yes I don't think it will be hard to do an ability like that, I'm more than happy to implement it! I think current single lane will most likely go into the Tutorial section, and the actual rhythm game will consist of 3-4 lanes where you need to press arrows. UI is what I enjoy working on, so I'm super glad you liked it!!