After being very frustrated with this problem for a long time and constantly rebooting to restart syncing I finally found the fix for this a while ago:
- Launch Activity Monitor;
- Search for processes called "bird" and kill them (sometimes there's just one, sometimes there are multiple);
- iCloud sync will now resume normally.
Of course it's completely ridiculous that Apple still can't get this working right after all these years, but at least this saves a lot of frustration, rebooting etc.
It does seem that this problem is occurring a little less often in Ventura than in previous versions of macOS, but it's still happening from time to time for me.
Edit:
in the Terminal should also work and is a bit quicker.