I'd avoid rsyncing the datastore.
I only tested backing up a few containers as I've currently no spare SSD to dedicate to datastores. But both removable media and NFS worked for me.
For syncing to a datastore on removable media, you need to manage the datastore after inserting media and prior to removing it.
AIUI `proxmox-backup-manager` provides for creating new datastores but doesn't currently support hooking up an existing but 'offline' datastore. You have to do that by modifying `/etc/proxmox-backup/datastore.cfg` and restarting the service.
Re NFS backed datastore, I did encounter some issues initially (EACCES?). I never made any notes so it couldn't have been too perplexing :-/ If I remember right all I did was use an NFSv4 export with no_root_squash. The root permissions were indeed required for the initial datastore creation. But once up, I think it is only backup user (34:34 on debian) that needed write permissions.
Mount the NFS export in the host filesystem e.g. '/srv/nfs'.
Then in PBS, create a new datastore & set its directory under that mount point. e.g. `/srv/nfs/datastore2`
In the new datastore, create a sync job to pull your existing live backups. Or have your PVE host backup directly to the new datastore. No need to edit the datastore.cfg directly.