With more gratitude and appreciation than I can adequately express, a friend of ours has diagnosed the issue and guided me to the fix, and search appears to be working again. This friend has asked to remain anonymous, so a huge thank you from me here without naming names.
Due apologies to everyone for letting it languish broken for so long. Will post another post about why that was here shortly.
Unknown: If the search indexes are accurate. As of tomorrow (because the indexer runs overnight) please let me know if it's not finding things it should. My default test, "pizza" is turning up hits from Dec 2020 so I think it's fairly up to date.
Edit - indexer is running properly, hits showing up from yesterday.