I may just not be accustomed to the side-scroller bullet hell genre, but this mainly felt mechanically off too me.
The size of the projectiles is too small while the player hitbox is too big. Each shot has to be fired individually at enemies that have to much health.
The background is also distracting from the enemies and projectiles.
Instant fixes I would make is lower the enemy health, make the enemy projectiles larger, shrink the player hit box, and make it so the shooting function fires if the key is pressed at all instead of if it was pressed that frame.
It sounds like you had a lot of fun making this so I hope you continue to hone your game design skills!