I really like this concept. I haven't seen many first person sort of immersive sims recently, especially not for this niche. I do have some possible solutions for a couple bugs in the game jam version.
For fullscreen you might wanna check out project settings > display > stretch, and make sure your UI nodes are using anchors so they stay fixed to a point on the screen. Viewport scaling might be useful for the low-resolution style, but both modes work.
For the music slider it might not be connected to a function, so for example the value_changed signal has to be connected to a function in the editor or with Signal.connect().
Movement is faster when moving diagonally, if you call normalized() on the player's movement direction/vector it should fix that. Movement is also pretty slow for a map with lots of long open hallways, but speed is an upgrade so it's not as much of an issue.
Overall a decent experience, straightforward dungeon crawlers. Always cool to see more games made with Godot.