I love schmups! I think this was a really well-made schmup, there is good progression with upgrades and the upgrades were pretty decent. It would've been great to have a little more variation on the enemies and a unique upgrades instead of just a double shot (other upgrades were just stat increases such as fire rate, speed(?), and extra lives).
Maybe local coop could be implemented in this game, like player 1 uses WASD and space while player 2 uses the arrow keys and maybe enter or shift (or maybe the mouse for movement and left click to fire, but it seems a bit unfair to the other player).
While I was testing the controls, I accidentally closed the game while using left ctrl and WASD (wasn't my brightest moment). So if you decide to implement coop with two players on the same keyboard, maybe change the fire button so it wouldn't be left control.
I love the message that the end when you win, wasn't expecting that. I really thought that this was an endless side-scroller. So it's nice to see that there was actually an ending implemented.