In style of the “XY problem” I wanted to suggest either Voicemeeter (Windows) by VB-Audio, it probably can do balancing too; or a hardware mod on an AUX cable (wouldn’t it be as simple as soldering a potentiometer or a resistor in series?). I guess technically one could solder a resistor inside the headphone ear-piece on the wire that goes to the speaker and prior an experimentation on a cheap cable would be reassuring.
I wonder how does the equalizer work for you now? Does it really allow per left/right channel settings? @draeh