Apple Mail is stuck at Indexing....

As in Apple Mail is stuck "Indexing…" - Apple Community

The symptom that bothers me is that Mail search does absolutely nothing. I put a word in the search box, hit return, and the same reverse-chronological order of messages in the message viewer remains. It is as if I didn't do a search.

The other symptom is that the activity window says it's still indexing:

I've repeatedly attempted the indexing a number of ways over the last month or two:

• Rebuild the Spotlight index on your Mac - Apple Support
• Rebuild mailboxes with Mail/mailboxes/Rebuild per https://nektony.com/how-to/rebuild-apple-mail
• Rebuild the Mail and Messages Spotlght by deselecting and selecting it in SystemSettings/Siri&Spotlight/Maile&Messages
• mdutil -E / ; mdutil -i off / ; mdutil -i on /
• (all with restarts inbetween)

I've seen hints that indexing "could take hours" for up to "thousands of messages." What is the longest time I should expect indexing to take? Is it longer than a week?