mds processes are constantly crashing
Okay, I think the issue is resolved.

The culprit turned out to be an old piece of software from 2011 - Tags, for Mail (http://www.caseapps.com/tags/).

It installed an mdimporter into /Library/Spotlight/.

After reading up on the new logging system and figuring out how to disable obfuscation in order to not get those &lt;private&gt; entries in the logs since those don't help me see which files are being accessed by the md processes,

I ran log stream --style compact --predicate '(process contains[c] "mdworker") &amp;&amp; (subsystem ==[c] "com.apple.spotlightserver")' --info --debug and watched it.

It kept hanging up in a particular directory, in ~/Mail/V6.  I went looking through the directory tree looking for permissions/uid issues -- nothing.

I re-ran the command with tee, so that I could tail it and grep out interesting parts to investigate.

Then I noticed that it seemed like _TagsMail.mdimporter was being mentioned in each line where mdworker was being called to import an email.

An excerpt:
2019-05-08 21:36:07.002 Db mdworker[53808:1463e7e] [com.apple.spotlightserver:Server] sandbox profile activated time:0.01667 uid:501 profile:mdworker cache:/var/folders/xb/25w333f48xl1ns0059bd0kxh0000gn/C/ temp:/var/folders/xb/25w333f48xl1ns0059bd0kxh0000gn/T/ home:/Users/B**** lib:/Users/B****/Library M:(null) MV2:(null) typeExtension:0 bundleId:com.gravityapps.TagsMail

I looked up tagsmail.mdimporter and the first web page hit is "_TagsMail.mdimporter prevents Spotlight in Mojave to finish": http://www.mnott.de/_tagsmail-mdimporter-prevents-spotlight-in-mojave-to-finish/

Interesting.  I moved the mdimporter plugin out and immediately saw progress being made, rather than the same roughly 10x files being attempted to index continually.  

Rebooted the machine, created a test file on the desktop, searched for it and saw it pop up almost immediately.  Good sign that the indexer is working properly.