Is there a convenient way for me to use my other macs audio output?
Sure. Since you can AirPlay system audio there are tons of solutions for this nowadays, any program that can act as an AirPlay receiver.
Many of these are commercial (like AirFoil and AirServer), not exactly made for your use case (media centers like Kodi) or  terminaly/somewhat fiddly (shairport-sync).
There should definitely be streamlined enough freeware/open source implementations out there that aren't outdated and abandoned, but I don't know of any. There's a list on the open-airplay github that might be worth a look.

Last time I needed to do this I used the old school method of soundflower plus a client+server to pass audio - Soundfly is free and still works fine. Can act up occasionally but more robust/set it and forget it than AirPlay in my experience.