Guys I might have found the issue.
I recently started learning game development and I just found out the issue might be Godot itself.
While following along some basic tutorials I experienced the same strange bug I see on BeatMagic too.
There are two red (glitchy) bars at the top and at bottom of the screen when in full screen mode.
You could make a Beat Magic version with an older Godot version.
Or I could be available to test beta releases and give you guys feedback.
Anyways I know it's an older Macbook but 2020 wasn't that long ago, and I think Beat Magic should be lighter and available to everyone!