I think the final enemy type definitely displayed the potential of this idea! I think one thing you could improve is make it so that hitting an enemy directly took off some health to add a bit more risk when trying to get shot.
Viewing post in Bullet Killer jam comments
I was thinking the same thing when playing it, losing health when getting hit by the enemy would add more risk. No collisions with the enemy felt a little too easy imo.
Having more complex shot types (like the last enemy type) would make things more interesting too. Or even having one of the ships move in a sine wave pattern or something.
Neat idea though. Good job!