Thanks for playing! I've been meaning to type how I got some of this to work so here it is for anyone wondering.
1: for the file system
I got it to take in custom music using the FileAccess class on Godot. I had it .READ the file and used a variable to hold it. Main menu script had that function called when the button was picked to start it.
2: for the beat
I had my wireframe shader call for a function that used AudioEffectSpectrumAnalyzer which has a method that sets a Hz range to track, then I had it turned into a float. I clamped that float with a global variable to adjust on each use of the shader. That final variable was what got sent to the shader to thicken drawn lines. WARNING: Should probably not attempt if you have epilepsy or if someone with that could see the screen. There was a few times my screen looked like a strobe light having multiple strokes at once before I fixed it . Especially when tied to the sky shader which was a epileptic war crime at first.
3: The glow was just lucky! I set a world environment node on my map that added glow and some fog to the game. I did not expect the glow to look as good as it did. This is basically what I was looking at all the time till the final day of the gamejam.