Nobody's picked this up yet, so here's what I'd check, in this order.
First, the Activity section at the bottom of Preferences > Lightroom Sync. Give it a few seconds to populate and it shows what sync is actually working on or stuck behind, which tells you more than the counter does.
Then the originals. Sync uploads Smart Previews, and Classic has to build those from the original files first. Since these 4,000 came in through Import from Another Catalog, make sure the main catalog can actually reach them. If they still point at the MacBook's drive or a disk that isn't connected, the Smart Previews never get built and the counter just sits there, with nothing showing in All Sync Errors. Quick test: select a handful of the stuck ones and choose Library > Previews > Build Smart Previews. If it reports missing files, that's your answer.
If the files are fine, there's a hidden reset on that same preferences tab: hold Option and a Rebuild Sync Data button appears. It rebuilds the local sync database without re-uploading what's already in the cloud, and it needs a restart.
And for any last stragglers, the old "tickling" trick from the other LRC sync thread still works: make any small change to them, a flag or a tiny Develop tweak, and they usually go.

Nobody's picked this up yet, so here's what I'd check, in this order.
First, the Activity section at the bottom of Preferences > Lightroom Sync. Give it a few seconds to populate and it shows what sync is actually working on or stuck behind, which tells you more than the counter does.
Then the originals. Sync uploads Smart Previews, and Classic has to build those from the original files first. Since these 4,000 came in through Import from Another Catalog, make sure the main catalog can actually reach them. If they still point at the MacBook's drive or a disk that isn't connected, the Smart Previews never get built and the counter just sits there, with nothing showing in All Sync Errors. Quick test: select a handful of the stuck ones and choose Library > Previews > Build Smart Previews. If it reports missing files, that's your answer.
If the files are fine, there's a hidden reset on that same preferences tab: hold Option and a Rebuild Sync Data button appears. It rebuilds the local sync database without re-uploading what's already in the cloud, and it needs a restart.
And for any last stragglers, the old "tickling" trick from the other LRC sync thread still works: make any small change to them, a flag or a tiny Develop tweak, and they usually go.

Thanks for the reply David. I stored the catalog and photos on an external drive when using the macbook. I created the smart previews during import. I then copied the whole drive to my windows computer drive before importing the catalog and photos to my main catalog. I did try the rebuild sync data but it didn’t help. I have finally gotten nearly all of the 4,000 photos synced by constantly deleting all the photos in the all sync error photos and then copying them back into their respective collection. I will try the tickling trick for the remaining few that haven’t synced. I just hope this problem doesn’t keep happening for all future imports.

I have been a user of LRC cloud syncing since it first became available. Prior to 15.6 no problems for last couple of years. I was on a 30 day vacation trip during Sept. I took over 4,000 photos processing them on my MacBook Pro M5. When I returned home I imported the vacation catalog into my main catalog (136,000 photos). No problems with the import but when I loaded the photos into a collection and synced it to the cloud, the problems started. Prior to that I had over 36,000 photos synced and no problems. Last synced a collection just days before I left for vacation. About 200 photos synced but the rest just stalled and the sync counter did not move. I finally deleted all 36,000 of the previously synced photos thinking that might help but it didn’t. I then removed the 4,000 photos from the synced collection. Then tried putting them in another collection and synced that. This time about 1,000 were synced but the rest stalled. There were about 16 in the All Sync Errors collection but none indicated any kind of error. I ran a diagnostic report and that also did not indicate any sync problems. I have been screwing around with this for several days to no end.

Anyone else experiencing similar sync problems since upgrading to 15.6?

I am having similar problems, since 15.6 update. New photos added to existing collections in Lightroom Classic, or to new collections, are not syncing to the cloud. But if I copy previously synced photos to a different collection, or to a new collection, they sync back to Lightroom Classic as I would expect. I have only just begun searching for information about this but it I am sure it is not "me" so I am thinking it must be a bug in 15.6. Anyone else experienced this?

I have been a user of LRC cloud syncing since it first became available. Prior to 15.6 no problems for last couple of years. I was on a 30 day vacation trip during Sept. I took over 4,000 photos processing them on my MacBook Pro M5. When I returned home I imported the vacation catalog into my main catalog (136,000 photos). No problems with the import but when I loaded the photos into a collection and synced it to the cloud, the problems started. Prior to that I had over 36,000 photos synced and no problems. Last synced a collection just days before I left for vacation. About 200 photos synced but the rest just stalled and the sync counter did not move. I finally deleted all 36,000 of the previously synced photos thinking that might help but it didn’t. I then removed the 4,000 photos from the synced collection. Then tried putting them in another collection and synced that. This time about 1,000 were synced but the rest stalled. There were about 16 in the All Sync Errors collection but none indicated any kind of error. I ran a diagnostic report and that also did not indicate any sync problems. I have been screwing around with this for several days to no end.

Anyone else experiencing similar sync problems since upgrading to 15.6?

Regarding the syncing of the 4,000 vacation photos keep tickling and nudging.

When you were on vacation using the Macbook were you using a new empty catalog ? Are you sure that sync was turned off ?

When you say you finally "deleted all 36,000 of previously synced photos" do you mean you unsynced them or deleted them from LrC entirely ? What have you subsequently done with these 36,000 photos ?