To be honest I haven't used SoundSource myself, but one of the early users of the app I made talked me about it. He mentioned he could select the input/output devices and Mac wouldn't change them when connecting a bluetooth device, as it does by default.

It's not exactly what my app does, but I thought it could fix the auto-switch for many users.

I'll definitely update the post if SounceSource cannot be used to fix the auto-switch. Thanks for the heads up!