iPhone photos Sync stuck on "waiting for items to copy"

I've had the problem of syncing many ( > 90k ) photos via USB fomr a MacBook Air M1 (OSX 11.6.1) to a IphoneX (IOS 14.8). The photos are in a specific user-chosen folder chosen in the photos tab of finder syncing window lets call it photos-to-Iphone-sync

It stayed stuck on "waiting for items to copy" ( all night and even more ) and nothing was copied

After a lot ( many hours …) of discussion with Applecare level 2 the issue was identified :

• The system does not manage to handle either of these causes
  • Very numerous photos ( ~ 55k seems to be ok I'm slowly moving up by adding a few hundred at a time
  • Too long paths ( i.e. many subfolders)
  • Too many subfolders

I moved the folders a few at a time out of a large subfolder into the photos-to-Iphone-sync folder

This seems to be working - partly.

Problem : The photos are synced but the message "waiting for items to copy" still appears for a long time.

you see that photos are synced ok on the mac when the iPod Photo Cache folder updates a large file Photo Database

Check the modification time

 photos-to-Iphone-sync/iPod\ Photo\ Cache/Photo\ Database 

It seems ok to press eject icon and remove the iphone then

All these should not be problems on the sort of powerful machines with lots of free space involved here.

At least a message should warn about the limits of the system ( "too many files " , or " too many subfolders, and give hints to help " try including less new items at each sync " or " reduce the number of subfolders "

Any comments welcome ( possible involving large number of pdfs synced ? https://discussions.apple.com/thread/253333243)