@pert Ok, so after many days, reading hundreds of posts, 2 fresh installs, and a few hairs pulled out, I have solved the issue and can quantify in part what caused it.
In October 2020 Microsoft released Windows 10 v20H2. They did not widely distribute it and it is believed that this is because of compatibility issues. Microsoft fixed these issues in later builds of that version. Now here's the kicker, the slipstream upgrade is what causes the issue. Upgrading from v2004 to v20H2 works perfectly, but Microsoft does not require users to upgrade to v2004 first. So users who did not get the v2004 upgrade in the upgrade lottery had to upgrade to v20H2 from a version predating v2004 and that's where it all breaks down.
If you are unfortunate to have gotten caught in this trap be prepared to do a reset followed by an upgrade to v21H1, which is more stable upgrade path for pre-v2004 PCs.
Thanks so much for taking the time to post an update with your solution. I'll take note of this information to share with others who might report the same sort of problem. I'm very glad to hear you found the fix.