Photos App on iMac stuck syncing the same number of items - how can I determine what they are?

Currently syncing some 65k photos from my iMac Photos library to iCloud, and thence to iPad and iPhone. However, for days and day on end, the progress report at the bottom of the library app states "synching 6 items to iCloud", and nothing changes. How can I determine what the 6 items are?

Background:

• Mac is Sequoia 15.0.1, iDevices are 18.0.1.
• I have run "repair library" on my iMac Photos Library, successfully rebuilt.
• The internet access has no problem.
• The iMac and iCloud Apple accounts are signed in the same.
• The iMac Photos library, and the Photos apps on the iDevices all agree on the number of photos except the iMac has 6 more items than the iDevices.
• Any change I make in the iMac Photos Library (like rotate, import, delete) is instantly reflected in the iCloud Photos library, and then on the iDevices, so there is syncing happening, other than "the 6 items"
• There are no folders in the iMac Photos app sidebar containing "Unable to Upload" (as has been reported elsewhere with stuck syncing), or any other kind of flagging of errors/issues.
• All other iCloud features are working as expected, and there are no reported issues with iCloud.
• Activity monitor shows no Photos or iCloud related activity in terms of CPU usage etc whilst the Photos app is open and reporting syncing of 6 items.

What I need to determine is what are the six items that refuse to sync are. Any ideas?

Many thanks in advance - it's doing my head in!