> I'd like the audio input to be able to pick up desktop sounds on MacOS the way it works on Windows.

Even paid apps like Wirecast are having a hard time figuring out a workaround for that. I ended up using Loopback from Rogue Amoeba ($99), and it does the job absolutely beautifully. You configure your Mac's audio output to point to Loopback (which shows up as an audio output device), and then you use your broadcasting tools to use Loopback (which also shows up as an audio input device). You can do mixing, too.