Fellow Godot enjoyer is progressing fast!
Compared to last DD things really take shape, lots of new things you put in the game. Very nice!
First thing to encounter is the stutter that others already pointed out. I had the same issues (which im hoping i've fixed by now, but lets wait for the DD verdict on my game) and the thing im doing currently to avoid it is to use the ResourceLoader.load_threaded_request(<instance>, "", true) for all kinds of instances i will soon need. In your case thats mostly the enemies and various projectiles i guess. A proper loading screen might do that as well and has the bonus of not showing that grey background when starting a game.
Secondly, i died pretty quickly on the first arcade run, somehow instinctively pressed escape while already having the little death menu. So the death menu was covered with the other menu and neither worked at that point. Pressing escape again made the top level menu disappear, but still nothing worked and i had to quit the game the hard way.
Speaking of menus, the fullscreen option doesnt seem to do anything for me. Also is it set to exclusive fullscreen? Because that might have weird interactions with some peoples setups. I used that myself for my first demo and had some negative feedback about it. Normal fullscreen seems to work better.
Lastly a small gameplay nitpick: switching weapons gets me confused, since pressing 1 switches the weapon on the right, while pressing 2 does it on the left.
Keep going, you are doing well!