You mentioned deleting the collection items; check the deleted section in Lightroom Web (https://lightroom.adobe.com) & clear any incomplete sync items. Try these suggestions:
Apply a flag or rating to some of the images syncing in Lightroom Classic & let Lightroom sit idle for 30-40 minutes to finish syncing.
Export the images a test out of Lightroom Classic & at the export window > select 'Add to this catalog'. This will create a final copy of the image but without the masks & edits. Try syncing the exported > reimported images.