Hey I also beat this game! It was hard but I enjoyed every second of it.
-If the final boss of this demo is going to be the first boss of the finished game, it should be a bit easier. I'd make the acorn bits-bounce not random, so that you can rely on learning the pattern of the acorns.
-Entering doors can be a bit tricky sometimes, I suggest making their hitbox a bit wider.
-The FlameDrop move can be canceled by simply releasing the down arrow key, which is good, but I think there should be some more visual feedback for this so that I see why my spell doesn't cast.
-A lot of enemies make use of RNG, which gets annoying in the tower where there's a bunch of corridors with jumping rodents walking in an area that's just 2 tiles high. I suggest you either make their jumps timed or add an anticipation animation for when they're about to jump.
-Same for the jumping fish, showing where they are before they jump and shoot would make the game feel a lot more fair.
The fist waterfall is over a pit that kills you, people might not know you can't doublejump in a waterfall, so maybe introduce it in a safe environment.
I really like the way the gems act like noteblocks and play a little tune when collected the right way.
And I like that empty blocks fart.
Edit: but why introduce the waterfall mechanic in a secret stage?