Well, after six sessions with MS Support steering the ship remotely,
they've given up. The issue was escalated to "Tier 1 - the engineers".
They suggested several things: running the update on a system as clean
as could be - no unnecessary services, startup items, etc; uninstalling
a handful of old apps; updating drivers, and using the generic video
driver instead of the Nvidia one. (There is a TON of traffic on Nvidia's
site about upgrade issues seemingly caused by their software. It's
almost enough to convince me to swap out the card for an AMD one, but I
don't have one and that's too big a hassle...)

Their final suggestion, 'tho they'll leave the issue open for a while,
is installation from scratch. That would be a gigantic headache, but I
may do it sometime.

As I've told Support all along, I've little interest in new features. I
just want security updates. From what I read of the timeline for
Windows-as-a-Service, it looks like my version falls off the edge early
next year - no more updates.