Files not syncing successfully on one Mac mini

drumsfeld wrote: Do you have “Optimize Storage” turned on? No. Should I?

Do you have more than 10% free storage on your iCloud account? Yes

Where does your Mac's Photos Library reside-- in the Pictures folder of your internal drive, or on an external drive? or on a network? on iCloud drive?

All on iCloud.

No. This is not the way iCloud works. iCloud is not storage that replaces the storage on your Mac. iCloud Photos keeps copies of your pictures so that they can be shared with other devices; it doesn't replace or add to the storage you need to operate your Mac.

iCloud Photos is a synchronization service. When you engage iCloud Photos on a device, then the Library on that device is kept exactly the same as the iCloud Photos Library, and it takes up the same space on your Mac as at iCloud. So, for instance, if you take a picture with your iPhone, it is added to the iPhone's Photos Library, copied to iCloud Photos Library, and then copied to the Photos Library on you Mac and on each of the other devices that you have connected. If you delete a picture on your Mac, then that picture is deleted at iCloud and on all the other devices. 

Optimize has Photos keep smaller, screen-sized images locally in your device's storage so you can quickly scan through pictures without having to wait for downloads from iCloud. Those pictures and Photos' database use storage on your device, perhaps 20% of the storage of the full Library. If you require full sized pictures because you zoom in or edit some, then Photos will need even more space.

So, where is your iCloud Library on the Mac? …in the Pictures folder of your internal drive, or on an external drive? or on a network? on iCloud drive?

What is your storage on this Mac? At System Settings>General>iStorage, what does yours say? Do you have more than 10% free storage on your Mac? Yes. 836.38 GB available of 994.66 GB

You need at least 10% of your storage space on any device to remain free for the device to be able to move files around. Less storage than that means that normal operations, even deleting of files, becomes laborious. .So you are right at 15% free. In spite of the 10% suggestion, I've seen problems on my 1 TB MacAir at less than 150 GB free.

Has this problem always existed, or did it just start? Has been a recurrent problem for the past several months, yes. Not sure what triggered it. A reboot used to rectify the issue; now, that is no longer the case. Updating to macOS Tahoe did not help, either.

Here are some troubleshooting things to try:

• Restart the computer (As you've tired, of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

• Make a new user and see if the same problem recurs with it.  This is a bit harder, because a new user can't access a different user's stuff. So you'll need to connect the new user to your Apple account and you would need to move your Library out of your own user's folder up to the general "Users" folder. 
• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon. Since you're using iCloud Photos, you might want to turn off iCloud, perhaps turning off wifi, itself. When you reconnect, it’s possible that this will cause a re-sync that lasts long enough to make you nervous.

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

Let us know what happens…

I'm not sure what you're saying. I see "My files do not sync successfully" and "files on this machine also sync properly." What are you concerned about, exactly?

How about some more information?

• Do you have “Optimize Storage” turned on?
• Do you have more than 10% free storage on your iCloud account?
• Where does your Mac's Photos Library reside-- in the Pictures folder of your internal drive, or on an external drive? or on a network? on iCloud drive?
• What is your storage on this Mac? At System Settings>General>iStorage, what does yours say? Do you have more than 10% free storage on your Mac?
• Has this problem always existed, or did it just start? Can you think of anything that changed that might affect your device?

drumsfeld wrote:

836.38 GB is available of 994.66 GB. This is ~ 90% free.

That's good. Mac Storage usually reports "used of" rather than "available of," so I misread your numbers. In either case, as I said, you're good.

drumsfeld wrote: … How do I ensure that this syncing happens in a more timely fashion on this machine?

Well, try using Safe Mode, or making a new user, or the other things I suggested and see what happens.