Lightroom CC migration failed (without errors?)

Has anyone been able to troubleshoot where a Lightroom CC migration completed, but never completed syncing more than a small percentage (~5%) of the photos. I've been talking with Adobe support on and off, but so far they don't appear to have any answers or strong explanations of what's going on. They've tried things, but they haven't shown any suggestion of really understanding what issue is being run into.

So far, my experience has been the following:

• Start a migration (a Lightroom Classic catalog converted from Lightroom 5), but have it appear to stall. Adobe support recommends wiping my cloud account clean and starting over.
• Start a second migration after clearing most metadata conflicts in my library and removing ~30 unlinked photos.
• The second migration would appear to stall from time to time, but closing and reopening LrCC would restart progress of the migration. This appeared to be every 20-40,000 assets processed. (My catalog has ~110,000).
• After completing the migration and giving me what appeared to generally be a positive result, but many photos still had to "sync". Photos appeared to continue to come into Lightroom CC, but very slowly for a time. Maybe 5 in an hour. (I have stable, gigabit internet, so this was unlikely to be a bandwidth issue.)
• I reached out to support, who then did a few things including something that removed the unsynced files Lightroom CC. I was told the photos would pull down and sync. This did not happen. From what I could tell most photos, if it matters, are in the cloud per the space Lightroom CC reports available, but they are visible in any way to Lightroom CC. The web version shows them as unsynced.
• I reached out to support again and was told to use Lightroom Classic to sync the photos. I turned on the sync and it "uploaded" about 102,000 images before halting. Oddly, while it was "syncing" these files, little to no bandwidth was being used, so I'm not sure what syncing was being done. While the uploaded images in Classic don't appear to be changing, the blue text in lightroom still says "syncing." However, there is nothing happening in "sync activity."

And there I stand.

Any help or suggestions needed.