Just a follow-up on downgrading:
tl;dr: Downgrading to Android 11 didn't fix it.
I was able to get Android 11 installed following the instructions on Asus's Android 12 beta page. (Sorry, I can't post links yet but Google finds it if you search for Asus Zenfone 8 Android 12 Developer Preview Program. On that page, scroll down to "How to opt-out and revert back to Android 11".)
The resulting system version is: WW_30.11.51.115. Unfortunately, this did not fix mobile service. In case it's relevant, the baseband version is now M3.13.24.45-Sake_0000110.
I would like to try rebooting, removing/reinstalling the SIM, etc. under this version, but the phone immediately downloaded an update (WW_user_30.12.112.82) and rebooting installed that update. At that point I turned off automatic updates lol. Baseband version is now M3.13.24.55-Sake_0000110.
So at this point I tried all the usual things (remove and reinstall SIM, network reset, factory reset, etc.), but I still have no mobile network.