FlexRAID volume expansion

My main 12-bay NAS has two volumes, orignally created in an RN516 with an EDA500, so it's in Flex-RAID mode. I left them that way so that I could move them back if I ever needed to because the (used) NAS failed. But I've had thast NAS and a spare 12-bay for a while now and have full backup, so I think it's time to consolodate them. I know I can do it "under the hood" via SSH, but I'd rather use Netgear tools to insure it's in the form the OS expects and because it should be a lot less work. But I'm a bit confused on what's available.

The primary volume (data) consists of 6 8TB drives upgraded fomr 6TBs, so with a layer of 6x6TB and 6x2TB, The other volume, which will be destroyed, so then the new added drives, are all 6TB (the ones rotated out of data into the EDA volume as I vertically expanded both). The structure of the EDA volume shouldn't matter, it's being destroyed.

So, can I add multiple drives to a single layer of a volume via the GUI? The Netgear KB on this just doesn't go into that detail. I would think yes, but I also thought I'd be able to add the remaining part of the larger drives to the vertical expansion layer via the GUI and I could not. I want this to be all at once, not a bunch of separate single-drive addition re-syncs, BTW.

If not via the GUI, what about via SSH and volume_util? These are the options:

    -e  Expand pool
        TYPE is one of 'auto', 'expand', 'extend', or 'xraid'
        auto - automatically do all forms of expansion
        expand - expand to fill all space on all current disks (disk replacement)
        extend - extend pool to use available new disks (disk addition)
        xraid - grow pool to use available space on some current disks
          (partial disk replacement)
    -E  Flex-RAID expand
        NAME is the desired pool name
        D1,..,DN is the list of disk channels to use, zero-based
          you may specify the disk by controller and channel, so
          '0,1' means chassis 0, channels 0 and 1
          '1:3,1:4' means chassis 1, channels 3 and 4
    -v  Vdev expand pool
        NAME is the desired pool name
        LEVEL is the desired RAID level (0,1,5,6,10)
        D1,..,DN is the list of disk channels to use, zero-based
          you may specify the disk by controller and channel, so
          '0,1' means chassis 0, channels 0 and 1
          '1:3,1:4' means chassis 1, channels 3 and 4

But the "pool" seems to be a volume (data) not a volume layer (data-0 or data-1) and it's completely unclear how one would use it to expand one layer of a dual-layer volume, if it's even an option. And as with the GUI, I could not use this to expand the data-1 layer after a 6TB drive was replaced by an 8TB and data-0 did a re-sync. Or at least I couldn't figure out the right syntax.

I suppose I can just proceed and see what happens. But I don't want to screw up the data volume completely and want to be prepared if I have to do it manually, partitioning the drives and adding the partitions to the OS, swap, and data RAIDs with mdadm.