About the movement speed, it's fine during every other part of the game, but it can make backtracking feel sluggish. I didn't mention it before, mainly because it's a tricky problem that's difficult to address in a lot of games. I think it's just something you'll have to coexist with, since it's easy to add too much to an only slightly tedious problem and turn it into a frustrating or annoying mechanic for players. There are things you can do to mitigate it. I'd look at horror games; they usually like to add spooky stuff to help fill in the dead time without actually affecting the player, also smaller/denser rooms could help. Personally, don't think it's a big enough deal to worry about; it's good enough to just avoid making the problem worse, like with large rooms or hallways.
I'm also not an expert game dev or anything, so take that with a grain of salt or something. ;-;
Also, while I really liked the ending and the "oh s***" moment when it didn't take damage, it didn't seem to be much of a threat once you get farther away from it. Even getting hit, it's easy to figure out the attack pattern on the spot, and getting shot at from off-screen wasn't as interesting when compared to the other enemies. Again, horror games are really good at this; they'll adjust the speed of the "chaser" depending on the distance from the player so that it's always just behind you. Might not work well with a projectile-based enemy, it can sometimes feel like trying to juke an aim-bot, but giving the final enemy more presence would have been nice, although I get the time constraints and difficulty of making it more complex. Still a great chase sequence, making the exit a really loud and memorable door-like thing made it the first place in my mind that could have been the exit (although that might change depending on the player, so playtesting would better tell you if it was obvious enough or not). I also appreciated that all you needed to do was run, sometimes horror games will add puzzles or other things into a chase, but that slows it down and makes it feel a bit cheap.
There were some little effects that I really enjoyed, the way the final room's walls rotated made it feel like a large moving machine. Small effect, but really effective. Really, the atmosphere of the game was nice, it changed when it needed to, and had good transitions or turning points that made the changes make sense. Especially during game jams, some sequences in games can feel rushed or come out of nowhere, so things like [hearing the goose honk off-screen -> becoming alert, -> and then seeing the enemy], [scanning big power core -> shooting it enough time to make me worried about ammo -> final enemy], or [normal floor -> entrance to dark unknown room -> seeing snake enemies go invisible] were great at setting the tone using audio cues, buildup, or enviornmental changes without just thrusting you into things with a hard cut transition or a cheap attack.
Art wise, things felt fairly consistent, although the UI felt a tad inconsistent with the rest of the game. Consistency is mainly what makes things look good, even without super detailed or complex sprites. Things like line thickness, outlines, and clear silhouettes are what I'd look out for. The UI just needs to have clearer shapes; you have a bunch of different modules that make up the UI, but the shapes kinda blend together, and some things look a bit wobbly. Also, texture-wise, some wall and floor textures were a little rough, for the same reasons as before, I'd recommend taking a look at minecraft's textures or any resource packs since they're really good about that kinda stuff. Sorry if this is a bit abstract; it was the only thing that really stood out as less consistent, but otherwise, it didn't affect gameplay by being hard to look at, which is the most important thing, so great work.
One last note about enemies, I noticed that some died a lot faster than others, not sure why. I suppose it probably had to do with hitting a weak point or something, but some of them would just drop dead without much of a fight. Could have been a bug or maybe a hidden bleeding mechanic, not sure, but if it is intended, it'd be nice to have an audio or visual cue of the enemy being critically wounded. It's a small and insignificant detail, just something that felt a bit odd ever once in a while, but was never a bad thing.
Lastly, showing the hp and oxygen but not showing the bullet count was a good idea. Not knowing your hp and just relying on screen effects or some other ambiguous cue gives it less presence, which would have made deciding between taking healing items or not a much more guided decision. Seeing your hp drop, or oxygen quickly reaching zero, helps add pressure by letting you know when you're going to die, and can activate the clutch gene in players, which leads to more exciting gameplay and avoids making deaths feel unexpected or unfair. This may be kinda obvious, but I wanted to mention it since you could have chosen to hide this info as you did with ammo, but it would have had the opposite effect, since you can be prepared to run out of ammo and have extra in hand, thus making it feel more fair when it does happen while also adding to the pressure and just a tiny bit of complexity to spice up combat. It might not work in other games, but it had a great effect here.
All in all, these were mainly small nitpicks I had, and considering this was a game jam, I'm surprised there were so few of them. Hopefully, some of my rambling was interesting. Anyway, good job on the game, I had a great time playing it! :D