Hey this is a really cool concept! There's a lot of really nice things going on here, and I love the music and visuals.
I feel like one major quality of life improvement you could make to make the controls feel a lot better is to utilize the move_and_slide() function (or something similar) to make the character feel less sticky and to give the game more momentum. Eventually I gave up on the second level because I would run into the ceiling or a wall and just get stuck, even though it feels like I should be shooting upwards along it. It was just a tad frustrating. The move_and_slide() for KinematicBody classes in Godot is designed to prevent collision objects from getting stuck like that, so if you're moving at a diagonal angle and run into a wall it will still let you slide along it at the vertical speed of that diagonal. Hopefully that makes sense.
Also it would be nice if the cursor was locked inside the window to prevent misclicks outside.
I definitely appreciate you making this available for linux though, the concept sounded interesting and I really wanted to try it! ^_^