iMac, iCloud Keychain checkbox won't sync
Older iMac (late 2013) I recently upgraded to max allowable macOS, Catalina 10.15.7. Upgrade went reasonably well from Sierra --> High Sierra --> 64 bit upgrade on all apps --> Catalina. All was fine for about a week, then went into system preferences, iCloud --> and noticed Keychain was not checked (when it had been checked on setup). The Keychain Access app was populated on the system at some point.
When trying to activate Keychain sync with iCloud checkmark on the iMac, I got various boxes about needing end-to-end encryption updates. In that process I got boxes asking for login-admin ID's for my other Apple devices, i.e., MacBook Pro, iPhone. It was a very slow process taking 5-10 minutes between the ask boxes. Then, It started to loop the same ask boxes one or two cycles, eventually ending up with iCloud Keychain unchecked. Now I'm getting a red dot on System Preference dock icon after start up saying... Update Apple ID Settings (caution triangle). When trying I get caught in the same loop.
I Went though 2 days of suggestions here and other websites, nothing worked. Hesitant to go into Keychain Access app --> preferences --> "Reset My Default Keychains" for fear of using 10-15 years of password data.
For the first time in a decade I called Apple Support. At their suggestion over 2 days, Disk First Aid, Safe Boot, Catalina clean reinstall. They finally gave up and suggested Genius Bar, or re-install High Sierra.
The online consensus seems to be a corrupted Keychain iCloud verification file, but there's no solution for fixing it. Any help would be appreciated
iMac 27″