So, after leaving the computer on for almost 24 hours, there is still no change - Search in Photo still doesn't work.
I see a Reddit posting complaining about the same problem - in that case, the problem corrected itself after about 20 hours. Since I have a lot of photos, I imagine it will take much longer for my photo set to be "analyzed". Same as for this Reddit user, I can create Smart Albums - doesn't that suggest that indexing is not the problem?
That said, I did re-index using the provided instructions. No change.
I see that using Smart Albums does accomplish the searches I would want to do. Thank you.
On the other hand, it creates additional albums that I have no interest in keeping and they will soon be junking up my albums list. There doesn't seem to be any way to delete Smart Albums. I can only see them (or any other Albums now) by Photos>View>Collections>Albums (whereas previously, Albums were conveniently visible in a panel on the left-side of the Photos screen). It doesn't work to right-click on any Smart Album and they can't be grabbed and dragged to Trash.
So, two new inconveniences - can't see Albums in side panel in Photo, and can't delete the Smart Albums I create for quick searches.