Smart idea! I like your implementation of the "only one button"-controls: felt very intuitive. I also really enjoyed the atmosphere, the time you put into understanding the lighting in Godot was well worth it. ;) I am not entirely sure, so I'll ask you: Is there any reload time? I felt like there's is a small pause after a while, if you spam-shoot too hard. Either way, I think that could be a good tool to increase the tension: when you're not able to shoot a zombie before it reaches you (even if it's a full screen length away!), that guarantees that you've to move back, further emphasizing your main mechanic. You could even go a step further and couple the reloading to movement: so you shoot all your rounds into that nasty zombie guarding the key (really funny btw! :D) and than have to backtrack a while in order to generate the final bullets required to put it into eternal rest.
My godot game: https://njamster.itch.io/wonkey (playable in browser, not mobile friendly)