Thanks for everyone's help (and enossified, def appreciate suggestions like yours--I was hoping/praying it was something easy/stupid that I had overlooked, but sadly that was not the case, Output was unmuted and audio was functioning normally on all other apps.) The problem resolved itself, but in a very unsatisfying & disconcerting way: it just...started working. I had posted my last comment--that I'd gotten it to work by routing Logic through Loopback--and then just left my computer in a frustrated huff. I had scheduled a Zoom appointment with a local studio tech who specializes in such problems, and didn't have a chance to try any of the suggestion people left here before he called a couple hours later. Sure enough, as these things go, when the tech called, Logic was all of a sudden working again. I can't think of any rational explanation for any of this, other than some weird thing like Logic's internal audio mapping was scrambled and leaving it on connected to Loopback somehow "reminded" it about the existence of the proper Output routing. Who knows. But got my blood pressure up for 24 hours anyway, and hopefully the issue doesn't return? Definitely unnerving that a serious issue appeared and then vanished without any specific fix on my part, but ¯\_(ツ)_/¯ .
I did end up talking to the tech and he recommended installing Apogee drivers on non-silicon Macs running Big Sur with this method:
(booting computer in Recovery Mode, running Terminal, and then running command "csrutil disable" and rebooting before installing drivers). He said he's seen a lot of Big Sur macs have funky issues with Apogee stuff. I did that and now (knock on wood), Symphony interface seems to be working with Logic. Thought I'd share. Really appreciate all the help, thanks everyone, and hopefully the issue doesn't recur. If it does, I'll be back here!