You could take a look at This
I’ve been migrating 4 nodes to new hard drives. I’ve tested multiple methods, but not rclone yet. Currently waiting for the biggest node to finish te sync, to start the last two. Only 700gb to go 
I’ve taken the nodes offline at the start of the sync. This speeds up the transfer, and doesn’t require you to “keep up” as you say. Nodes can be down for ~12 days before they get suspended. After that, it takes even more time than that to get disqualified. My biggest node has 7TB of used data, and the move so far is taking 8 days. I expect it to be done tomorrow by the end of the day, well within the margin of 12 days.