Actually you can do an online sync, also it’s recommended to reduce the allocation below the usage - it will stop uploads to your node, so the final sync would be much shorter.
But as others said - your node can be offline up to 12 days, then it would be suspended (again, no uploads) and you would have a week to fix the issue. And yes, the longer the node offline, then more pieces could be removed later due to a repair to other nodes.
Please do not forget to add a --delete option to your final rsync command to remove deleted files, otherwise you may end with corrupted databases.
Also, many reported, that rclone works faster,