Really nice one ! I especially liked the pixel arts, the background art/effects fits well the ambiance. Aswell as the musics (menu and ingame) that respectively brings the calm of space and then the uplifting rhythm of battle.
About level design, I feel like you could make the last boss a little bit stronger. As player we do feel stronger with levels, recruiting spaceships, but I felt the last boss level a bit easier than the 1st or 2nd one. As every player comes at the last level with the same build you could make it a bit harder in my opinion :)
Thanks I had a lot of fun !