I have no idea why, when I rebooted after 6 hours, it was back to normal. Regardless, I took that opportunity and I did as someone suggested and went to Mac HD > Library > Extensions. [not the user Library]
Not seeing any of the specific ".kext" files others mentioned, I deleted any .kext file older than 2017.
Not only is it a bit "snappier", booting quicker, but my Siri is healthy again (...and the villagers rejoiced. Apple never did reply). Previously, it errored the 2nd time it was accessed, often not working at all. This was a standard failure for the past several weeks.
My friends (not developers, no beta) started having the same issue (freezing on boot-up) yesterday. I'll try a Time Machine restore from 3 days ago (last known good boot date). If successful, I'll delete all old kext files using Finder.
I'm wondering if too many Time Machine and Migration Assistants from the past (in Apple years that's 5 years when they stop caring) results in old conflicting files are never discarded when OS X is updated.
If not, I'll use the suggestion of command-R and Terminal to delete the older kext files to see what happens. I'm far from knowing UNIX, but I think I can do a few safe commands. I'll report back with the results.
I miss Steve's attention to detail, too.
(Don't get me started on the Photos icon.)