I'm drilling into this question because my next step is a PCB board for my project, and I don't want to create something that is noisy. Naively, before running into this problem, I would have created a GND plane and connect every ground to it. I've modified my breadboard and now all the audio GNDs are connected directly to the TRS jacks and disconnected from the other grounds and this works quietly.
Given I'm using 5V to run the LCD screen backlight and 3V for everything else (Bluetooth HC05, LCD1602, and switches), should I connect all belamini GNDs to my ground plane? Or should I have a separate GND for 3.3.v, and connect the 5V GND for the LCD backlight directly to P1_22? Or only connect one GND on the bela mini?