Honestly, I don't know what it is about Plague Doctors, but I love them. I loved the idea of using the instrument to make different cures for the colored plagues. I'm excited to see if you'll take it further with note mixes, or something. (Purple plagues perhaps?) The music was great has that kind of whimsical forest vibe, very catchy. I liked the design of the of plagues and doctor, and the keyboard. I like to believe now that there is a eccentric plague doctor out there blasting illnesses with a piano guitar. The fact you kept the colors on the screen makes me happy, because I don't usually play rhythm games.
Having the signs as a tutorial was a good idea. Kept in the immersion of the game, while explaining to new players. I would mention what the jump button is, however for continuing the signs. Even if it's obvious, some of your players won't be use to computer game keyboards.
The only two things I noticed that I didn't really see anyone else mention was that I could pass through some enemies by jumping on them with the wrong color. I noticed it with one of the blue fungi at the end of level 1, and the red monster on level 2. I would test the rest of the enemies, but that will take me quite a few days because I'm bad a jumping puzzles, rofl.
I did notice the winds at the end of level one won't blow the player the whole way either, I wasn't sure if that was intentional or not, but I'm writing it just in case.
I'm late to the party playing this game so, I was rereading over comments to make sure I didn't repeat what you already heard. I saw you had issues with the boss, but you fixed it almost as soon as you learned about it. I want to say I am impressed about your timely response and taking feedback. It's something I have to improve on myself, so I try to keep my eyes out for how other devs do it.
All in all, excellent work! I hope you do keep polishing and working on this game. I want to see what other ideas and things you'll add in now that you have more freedom.