I really liked this one impressive project for only 72 hours!
The game feels fun the movement and the auto fire are nice and good the only thing that I was totally not feeling was the pacing of the thing. Having one enemy type is gonna get repetitive for obvious reasons but the game does a good job mixing it up with different formations and the one enemy there is has a challenging enough bullet spread for it to be fun for quite a while. However considering the small amount of content in game I feel like the final boss is kind of a long ways away into the level and the whole thing would've benefited from being shorter. This also might just be me not playing a lot of SHMUPs and being bad to be fair.
The design is great there are three upgrades and I think they're nice simple and balanced really well they add a fun choice that's made throughout the game and allow the player to experiment with different builds. I think the heart is probably the most lackluster upgrade since it doesn't change how the game is played quite like spread or damage but I'm not dreading it's inclusion or anything.
As far as visuals go the game looks good everything has this nice simple pixel style with a nice mostly green color palette which lets the red bullets really stand out. I like the jetpack guys little particle effect. I think a particle effect for the dragons getting hit or dying could add a lot to the games feel as well. The music is good and I think in a SHMUP it's generally a good thing to keep the music at the forefront of the mix so the game doesn't just sound like a bunch of explosions. However I think you could have added more sound effects and considered which sound effects are really important for the player. For example the main sound effect in your game is the sound of the gun shooting which is a fine sound effect to hear (albeit a bit grading on the ears after a while) but it's not really something the player needs a lot of feedback for since the gun is basically always shooting. I think it's much more important for sound effects like enemies getting hit and dying to be the loudest thing going on since those are more important moments. I hope I explained that well.
Overall really great entry especially for the short time period my main criticisms would just be pacing and more consideration for the nuances of sound design. Otherwise fun game!