For a first game, this felt well constructed. I appreciated that there was some variation in scoring that increased the tactical value of charged shots, and it was nice to see a range of behaviours from the boss as well as multiple enemy types.
I found myself wishing that there was audio.
Glad to hear that you're having a positive experience with Godot, and if you add the powerups you were intending, I think that would make for more interesting gameplay that could sustain longer levels.
Thanks for doing a Linux build!