Thankfully, a buddy of mine pointed me to SetupDiag, which parses the logs to get you more info. In the end, the update was failing due to an accelerometer driver (!!).
A fresh install with my original media failed in the same way during updates.
A fresh install (on a spare HD) with the media creation tool worked once, but crashed and burned after the first time I ran Windows update. I might try again, but I'm a little worn out.
I actually wasted hours with MS chat support, but they offered no insight except running me through a fresh install script. Once I hit the end they started talking about BIOS updates. However that's an entirely different wall, because apparently my MB shipped with an update that's not recommended for my CPU, but I cannot downgrade. I'm now debating whether to shell out $$$ for CPU upgrade (not even to current gen), but the ones I want are out of stock and only being sold by iffy sellers.
I just come back to find my PC randomly rebooted overnight.
Force updates are debatable for security patches, but they're entirely unacceptable for new features.
Windows 10 is biggest garbage fire POS Windows version yet, because of their new update model. For the last several months, I've been unable to install the mandatory (for non-enterprise users) feature update patches. They fail to install with a cryptic error code and get rolled back. If I don't get this figured out soon, I'm not going to be able to get security patches anymore.
I should be able to get a version that's stable (enterprise licensees get access to LTSC versions that never are required to install feature updates), but they've denied everyone else. The best I could do was buy professional, which let me delay the patches for up to a year (instead of only a week or two). My understanding is MS laid off most of their Windows QA team, and replaced it with home users drafted to become guinea pigs plus telemetry, and the lack of it shows.