Sometimes you don't need much to make a great game. You really nailed the production part I think. You really prioritized well and put work where it matters : level design, enemy visuals, polish. Awesome result.
Game design is great, level length is perfect and having 3 hp is the correct amount of challenge. The range on the pistol is also well calibrated, it's the most important design decision you took. It forces you to actually dodge projectiles, but also gives you a bit of leniency for positioning yourself. This kind of design stuff often goes unappreciated in games but it's very important, so good job and great vision.
Also, the fast respawn and constant checkpoints. Sooo important. Jammers, take inspiration, I don't want to restart your 5 min games for 1 mistake. With that said, segmenting different challenges is very hard, I can tell you spent a lot of time on level design.
Small polish neatpicks :
- the train separation kinda looks like a hole. Maybe it could be transparent ? Idk.
- maybe enemies could've had a different color ? Sometimes they were hard to tell apart until they attacked. Could also be corrected by having more contrasted shapes. I understand why you kept all the projectiles though, I think it was a good idea (makes the bullet hell segments easier to read).