Phew, that was a hard one, good thing I have all that Touhou experience from my weeb days :D
I really think you got the difficulty progression down! It took me a few tries, but the progression from "Ha, more like the possible boss, am I right?" to "oh shit, oh shit, oh shit" was very nice. I especially liked the flipped controls one, caught me off-guard.
You mentioned not adding music because you suck at it - which is fine, and I can respect it a lot to choose just to use what you yourself can do, but I can't help but imagine how much some escalating, frantic music might add to the feel of the game. It's fun without it, too, though.
Same goes for sound effects - although I can't be sure if there being none might have to do with me running it in wine on Linux.
Which, btw, worked flawlessly, I guess that is standard for Unity games, but you could advertise it as working on Linux with wine too, if you wanted.
Overall, I enjoyed it! I think you really got some talent there for escalating encounter design you can build on for future projects.