Understood. It's just that I'd done all of those things, minus disabling the extensions, and they never kick-started syncing before. It wasn't until I did all of those things, but this time adding in the disabling of the extensions, and BINGO - sync started and everything appeared on each device as before.
I'll keep checking. I feel as you do that it's only going be a temporary fix. But, if it sticks...might at least have something to do with Extensions in general, if not specifically AdGuard. I'm just pinpointing AdGuard as the potential culprit because it's the only extension of mine that seeks to block things - content, website access, etc. So, I figure, maybe it's blocking access to iCloud.
Shot in the dark, I know...