Small update
So it's been over 4 days of file transferring. There are about 13.6 million files in the source buckets, and yesterday we passed 14 million file checks.
So I'm guessing that means it needs to do multiple checks per file. I'm hoping it's 2 checks per file, meaning the process is over half way.
The good news that most of the data has been transferred over (source shows 727GB used, destination shows 713 GB).
The bad news is that due to how the files are structured, it's incredibly hard for the sync process to estimate which files it still has to move over, so it does need to check them all one-by-one.
Ironically, me posting this image above would mean there's 1 file more that has to be synced over. But I'm willing to let it be a casualty of this war ;)