So this will be a bit of a unique comment but I actually played your game on Saturday before the jam ended too, so it's cool to see how the game developed from that original build you had posted. First off, congratulations on doing your first jam! I like the idea you had here and with a little more time to polish it too, I think you could really have something neat here.
The biggest suggestion I would make is limiting how far up the screen the enemies can spawn. I would restrict them to the bottom 30% or so of the screen as several times I had an enemy spawn right in front of me and already be out of bullet range before the next bullet spawned. I like the added effects you got in between the two builds too, gives it a more interesting look.