I haven't solved the problem, but I've figured some of it out. It is not actually all of my MMS messages, it is only those sent/received since October 14, 2023. Something happened at that point with RCS that coded the messages differently. SMS Backup & Restore claims to backup RCS as MMS, and thus can restore it, but something changed that is lost in translation now.
I am trying to load Google Messages and enable RCS, but presently it is unable to verify my phone number. When that gets worked out, I wouldn't be surprised if Google was able to decode those particular missing MMS/RCS messages