That whole thing was great, I really enjoyed it! Gameplay, art, audio, and design of the levels, enemies and boss battles, all seemed considered and worked out but simple enough to not be overboard within the scope of the project.
I came here from the r/Gamemaker post where you mentioned the lighting stuff you'd done and I think it worked really well. While playing it was fun to have to explore and memorise the immediate area to find where platforms were as they're stationary, and having most of the enemies with glinting eyes or stuff visible through the darkness was great as I didn't have to try and track or guess where the moving things had gone. Also turning on that light switch in one of the rooms looked awesome. Bar the lighting which I have a soft spot for, once I learned I had a jetpack and could double jump the movement was smooth and controllable which made the big open room boss fights really fun. As for enemies the shield bubbles were fun to play around as it forced a change in play style, and how the skulls work is just great. Finally I appreciated the screen shake on the last boss. As it hadn't been overused during the whole game it gave that final fight a nice kick. Lots of other stuff was also awesome.
There were a couple things I found a little frustrating, hopefully even if you disagree with them it'll give another players perspective. The black box enemies that charge you down seemingly had no counterplay to them in some places. Sometimes they were placed in small corridors with not enough space to jump over them. Also them being mostly black made them hard to see in the darkness and with the speed they leap at you I didn't have any time to react. Then with movement a couple things, the lead up frames to the jump make it feel nice most of the time but pressing jump while moving to the end of a platform and the guy just walking off before the frames are done without jumping doesn't. That one's a bit of compromise I guess, having it a few frames shorter might make it feel more reactive while retaining the nice animation. And falling through platforms was a bit tricky at times especially when you're concentrating on jetpacking around a mob of bats. Personally I never found a use for pure crouching so would just use holding S for falling through platforms so it's only one key to worry about.
Thanks for making the game and sharing it, apologies if you weren't looking for this sort of feedback but hopefully it's useful in some way!