As your first game, this is fantastic.
It has a lot of potential. I especially liked the last few rooms. Showing the player a collectable in a different screen like that is clever. And the last puzzle is also quite well designed, except for the possible "softlock" if you fail to grab the collectable as you descend.
I second everyone else when it comes to the coyote time. However, I wouldn't change a thing when it comes to the little pixels all having collisions, except for maybe smoothing up a few surfaces. That's very "game boy" in a deliberate way that's hard to explain, there's just this vagueness to everything while the fact that you interact with every single bit of rubble in the ruins grounds you; it feels very intimate, and it makes you wonder what this place used to be. I think that feeling, in that pretty red palette is at the core of this game, and that if you expanded on these ideas you could easily turn it into something that gives Celeste, Cave Story, Noita vibes.
Great work all round.