Photos app stuck uploading for a week or so

How I fixed it:

Thanks to leonie's post here this is the order of operations and it needs a lot of work:

1. Open upload cache according to leonie's instructions (using "find" etc)
2. See especially all videos starting from the oldest and try to locate their respective originals in your library. Difficult. The dates might help you.
3. Export them from your library "as unmodified originals" (file menu) to a temporary folder that you will create for this purpose. Note their dates (because this information is lost). Delete them from your library. Delete them also from the "recently deleted" folder in your library.
4. See also photos in the cache, if you see something awkward (broken images, crazy file formats etc) do the same as above for them too.
5. Shut down Mac - start in safe mode - shut down again - turn on normally
6. Open Photos app, scroll down in photos, see if uploads/downloads sync has started. Might need to wait a bit, but the safe/normal boot sequence above helps start the sync.
7. Double check that these files are now missing from your upload cache (using "find" etc)
8. Repeat all steps above as necessary if sync gets stuck again
9. When your sync finishes, import your videos and photos from your temporary folder to your library one by one so as to check if they can be uploaded. Convert any that cannot. Fix their dates once imported if necessary.

Arm yourselves with a lot of patience! This is the only way to fix this until Apple decides to make a decent Photos and iCloud product that doesn't get stuck and shows update progress, has debugging options, skips problematic files, keeps a record of upload errors, allows user to force-sync etc etc. (Shame...)

Hey léonie, your comments are always very helpful!

Re restart:

Well I had restarted my Mac, but not in safe mode and back in normal mode like you propose.

When I first started Photos it said in the bottom something like "Last update March 25th"

Then it changed to "78.000images, 4,700videos updating..." and I've been seeing this for a couple of hours.

Then it changed to "Updating 1,255 items..."

I hoped that this will do the trick, but see below...

Do you have enough free cloud storage?

Yes my friend. More than enough. About 45% of my 1 TB is free.

You could try to check for the photos waiting to be uploaded by looking into the Library package in the Finder. This is described here: https://discussions.apple.com/docs/DOC-250002457•

Thanks for the tip. The list was 21 videos and about 1,400images long.

The list remained the same after the safe/normal reboot (from what I can remember by looking at the previews).

It's so long that it's difficult to be useful...

If the library is small, or we are just trying to upload a few new photos, we can also look at the most recent photos on the webpage www.icloud.com and check, which photos already made it into iCloud. But this will not work for a very large library, if we are currently uploading the complete library.

Well, there was video from year ...7962 in the iCloud Photo Library! Unless I time travelled with my dad some 6000 years ahead and he rides a horse, this must be wrong... I deleted it reluctantly (also from the bin - I have a copy to re-upload) and we'll see what happens. Reluctantly because if I edit the iCloud library from anywhere else than from my Mac, I don't know if the deletion will cause the Mac to think that it has an older library all together and substitute the entire content from the cloud. Hope it keeps timestamps for each of the numerous changes I've done since that video was uploaded in that crazy date...

After all this above, the next day morning Photos app still shows "Last update March 25th" and occasionally it changes to "78.000images, 4,700videos updating..."

I AM DESPERATE!