Nice job! Very nice vibe to the game, and challenging without being overly hard.
I did find a way to softlock the game, though I'm not 100% sure what I did. I'm thinking what caused it was that I had one alien left, shot two rounds, the first one hit a power-up ship that exploded and was crashing down as the second round hit the alien. The game softlocked and would not play fireworks/spawn new aliens, even though there were no aliens left in the round.