On Mojave, Photos has stopped syncing with iCloud in either direction. Apple support has dead-ended. Even opening the iCloud Preference pane and looking the the Photos sync config caused Preferences to beachball either indefinitely or for 15-20 minutes as it does god-knows-what. All other Photos clients (MacOS and iOS) syncs as expected and other iCloud app sync on this mac seems functional. Problem is limited to this Mojave Mac's Photos iCloud sync.
I'm throwing a hail-mary here because AppleCare has led nowheresville.
Back in November, on this same machine, I imported about 60K photos which I later realized that 30K were thumbnails of existing phots or other duplicate garbage. In December I used a photo-data de-dupe tool (Gemini 2) that removed these photos and the deletion propogated successfully across iCloud. However, since this completed all other Photo sync has ceased on this client. It is stuck in a state where it thinks it is either waiting to sync or starting the sync and doesn't progress.
If I can disconnected from iCloud and manually wipe residue that might be making this problem persist, that would be a great thing to try. If doing so would cause Photos to merge database so duplicates would be created, I'd probably prefer removing the Photos database locally and just downloading everything from scratch as long as I can be sure configuration was definitely cleaned so the problem would be removed after all my troubles.
Any guidance on how to dig more deeply in Photos-iCloud sync issues greatly appreciated!