Starting with robocopy is nuts. I'd have started with rsync and just let it run overnight. The peace of mind that it'd do the right thing and not needing to research flags are more than worth any speed tradeoffs.
I recently migrated from a synology to a diy nas, 8to to transfer, did it through rsync, took a few days but it ended up working nice.
I recently migrated from a synology to a diy nas, 8to to transfer, did it through rsync, took a few days but it ended up working nice.