This was surprisingly good - my first impression was that as an "experimental" game, it was retreading some very familiar FPS territory, but I think that the different wall effects were able to bring something really interesting to the mix.
I'm unsure whether it would make sense to do and would require playtesting to really assess the impact on gameplay, but I found myself feeling like it might be more interesting if attacks that weren't through walls did no damage.
I also wish that I was able to properly invert the mouse Y axis!
I did get some window flickering, and I wasn't able to move the game window to another screen - it would snap back to the screen that I didn't want it on after a frame. I wonder whether controlling window placement the way that you are is negatively affecting performance.
The game also tries to run maximised even when it's started in windowed mode at a lower resolution - that might be playing a part in the scaling problem that unless games mentioned?
Hope you had a positive experience using the Godot engine, and thanks for doing a Linux build!