"Ideally you'll find more value in the comments as Hacker News folks and Windows experts will drop in with better tools and strategies."

I don't know if this is better, but...an alternative is rclone, which uses rsync under the hood. It can also do parallel jobs and is restartable on any failed partial copy.

When I've had to do large Synology-to-Synology moves, that's what I've used. The multiple rsyncs in parallel really helps if you've got a lot of small files.