Used the Software Updater to add the groovy-proposed repository.
Selected and installed the ZFS packages (from Software Updater).
Installed a small package (using "apt install") to force a grub update.
Rebooted.
"df -h" showed root mounted on the correct filesystem (instead of the previous, truncated "rpool/ROOT/ubuntu_").
Yes!
Used the Software Updater to add the groovy-proposed repository.
Selected and installed the ZFS packages (from Software Updater).
Installed a small package (using "apt install") to force a grub update.
Rebooted.
"df -h" showed root mounted on the correct filesystem (instead of the previous, truncated "rpool/ ROOT/ubuntu_ ").
Thank folks!!