That is really cool!
I really enjoyed the boss battle, although it got a bit frustrating at times. When I figured out the strategy to farm overload in the star room, shoot primary attack for the first half and then spam secondary attack for the second half, it was much easier.
My main pain point is balancing. Mainly: low amount of i-frames, enemy being able to shoot you through walls and off-screen and enemies shooting at you as soon as you enter the room. But that could be solved with some minor tweaking, all the important stuff was spot-on!
BTW, was Risk Of Rain (the first one) an inspiration? The premise and graphics are similar. And the checkpoint/health restore objects remind me of Remote Caffeinator from Risk Of Rain 2