The pixel art for this game is incredible, something to study. The SNES vibes are immaculate, I love it.
I had a few issues with the gameplay, most notably the enemies being very difficult to avoid. Rooms are often cramped and their hitbox are super large, sometimes as big as their sprites (or maybe even bigger?). If your fireballs are not upgraded, some of them takes forever to kill (those red slimes!). And they constantly respawn... Being on a timer, I've found it often more advantageous to just ignore them, tank the damages and hope to heal it later.
I've also had trouble remembering which item could open which colored doors. I would have preferred all doors to be blocks with the item's symbol in it, like some of them already are. And I felt a map was sorely missing in the later missions (though I think you started the jam late due to finishing another one first? Think of it as a suggestion for the post-jam version ;))
I liked the idea of the separate missions, it gave the game a bit of a "randomizer vibe". Like you're never sure what you will get to play with this time and in which order. That's interesting. Though it evoked to me more of an "arcade" feeling rather than "minimalist".
Again, congrats on such a solid entry (made in very little time?) that oozes style!