It costs US $100 plus tax, it is hitting a mosquito with a sledgehammer, and Apple holds up several hoops one must jump through to get kernel extensions enabled these days…  but if all else fails and you cannot achieve this natively, I have done this in the past with third-party software: Rogue Amoeba’s Loopback.

Here is an example, yielding reverse stereo by just digitally crossing the wires over. The audio playing at the time of this screenshot was louder on the right, but in the bluetooth output pictured, and my ears, they’ve been swapped and it’s louder on the left.