What about if you remove edits without exporting (select, then image>revert to original) Does that solve the unable to upload problem.
Can you also clarify - if you safe out of preview, are you finding the newly saved image then won't import into Photos? Or that it won't upload? How are you importing? Drag/drop, or file>import?
When you edit in luminar are you "round tripping" (Eg "edit in luminar", then save from luminar and it appears automatically back in the library). If so, it might be that process that is going wrong. It is quite possible that process could go wrong for some images and not others. The round tripping mechanism in Photos is not that good. Also luminar is one of the most buggy pieces of software I've ever had the misfortune to purchase, and I don't let it near my photos library any more.
Where is your library stored? In you pictures folder or somewhere else?
Have you ever used and clean up, or speed up apps on your mac?
I'd also try running first aid on your fusion drive to check it has no errors?
Next please try the following:
1 - Create a new empty library and try with that library open (hold down the option key while starting photos, and click "create new". To switch back to your currently library afterwards, do the option key start again)
2 - Try booting in safe mode - does it still happen?
3 - Try creating a new user account and try when logged into that account.
These checks will tell us if the problem is system wide, or linked to a particular library or user account.
Note that to try point 1, you would have to make it the new library the system library - which would restart the sync to iCloud - which you've said you don't want to do.
Points 2 and 3 you could try without restarting the sync with your current library. Point 3 you would need to import some of the problem images into the new user account library, and try syncing that with iCloud. You could either use your own apple ID - in which case everything currently in ICloud would sync to the new account (it can all be deleted after the test). Or you could create a new temporary apple ID on your mac, and add it to the iCloud family sharing, then try with that.
