Previously I wrote about how I'd tried to create an app, but ultimately failed because I wasn't getting the results I wanted out of the macOS CoreAudio APIs.
Thanks to some excellent input from David Lublin I refactored the code to be able to switch easily between different backend audio APIs, and implemented a replacement for CoreAudio using AVFoundation's AVCaptureSession and it seems to work!
I'd still like to settle back on CoreAudio at some point, but for now I can rest assured that whenever the older versions of SoundSource stop working, I still have a working option.