It's not clear to me that anything is working improperly here. It seems to me that you are going to do lots of work and take actions when none are needed.
Why are you trying to Import or Synchronize folders that have your photos in them and are properly in the catalog after you did the Update Folder Location?
It is when new images are imported that the synchronize says there are ~12k images to import. I could replicate the behavior just by doing the sync process on the top level folder.
It is when new images are imported that the synchronize says there are ~12k images to import.
I don't understand.
You have imported new images, and then you want to synchronize? Why isn't importing sufficient? Why do you need to synchronize after import?
Also, that's the proper behavior of synchronize, it just counts photos in the selected folder(s). It doesn't attempt to determine if the photos are duplicates that should not be imported, until you click on Synchronize and actually start the process, then it will most likely find nothing to import.
Maybe I have the wrong work flow, but this behavior started after I moved a top level folder.
I transfer images from the camera to the computer with Bridge. After that I go to LR and synchronize the top level folder where the images were placed by the Bridge process. I then synchronize the top level folder that was originally imported into LR so that LR has the new images in the catalog.
The top level folder is camera make and model with shot date sub folders underneath.
Why are you using Bridge and not Lightroom to import, kinda causing you issues.
As Paige has stated what Synchronise does is count files then when activated will detect Dupilcates...
No particular reason, was just using Bridge before I started using LR. I'll switch to using LR to do the import.