I am encountering the same issue on my Macs, both of which are running Intel chips, one on 10.15.7 and one on 11.
Regardless of OS, all of my devices except for an iPhone are failing to sync via iCloud. I am unable to turn off cloud sync or select a different sync location within the App to even reset the iCloud login. It just sits stuck failing to synchronize. All devices are running the most recent version of the application.