Hey! I really dig what you're doing here (I like that part of the game feels like it's a bit of a Zelda II riff, with the downthrusts and upthrusts and some of the gel-like enemies). Yet the speed, structure, and aesthetic give the game feel a really strong and independent identity. I like it, and can't wait to play whatever iterations of this game/concept come after!
A handful of thoughts:
1. I think the contrast between the brick background and (especially) the disappearing platforms could stand to be increased, probably by darkening the background.
2. In the stage entitled "As Above, So Below" the jump onto the thin platform in the middle of the room felt unreliable. That is to say, it was hard for me to predict whether I was going to make the jump successfully or not, and it was hard to tell what exactly I was doing that separated a successful jump from an unsuccessful jump (in all cases, it felt to me like I was doing more or less the same thing).
3. The way collision looks, it often seems like my character dies before they come in contact with the harmful object. The hit detection may be a bit too punishing but it also might be smoothed over by the game waiting a part of a second to let me see my failure before restarting the stage.
4. If the projectile-shooters offered even the tiniest bit of animation to signal they were about to fire, I think that'd be neat!
I haven't quite finished the game. I may share more thoughts after I do! Good luck with your ongoing development. This is neat!