You can also do:
Detach first drive from rpool: sudo zpool detach rpool {Drive 1 PARTUUID}
Ensure rpool is still good: sudo zpool status
Attach first drive to rpool: sudo zpool attach rpool {Drive 2 PARTUUID} {Drive 1 PARTUUID}
Allow the automatic resilver to complete, monitor it with: sudo zpool status 5
Scrub rpool: sudo zpool scrub rpool
And monitor it with: sudo zpool status 5
Detach second drive from rpool: sudo zpool detach rpool {Drive 2 PARTUUID}
Ensure rpool is still good: sudo zpool status
Attach second drive to rpool: sudo zpool attach rpool {Drive 1 PARTUUID} {Drive 2 PARTUUID}
Allow the automatic resilver to complete, monitor it with: sudo zpool status 5
Scrub rpool: sudo zpool scrub rpool
And monitor it with: sudo zpool status 5
Doing the above apparently deletes the progress data that zpool initialize keeps, so it starts over from the beginning. If you don't delete that progress data, on subsequent runs zpool initialize believes it's already done, and exits quickly, so sectors don't get zero'd.
Then run:
gnome-terminal -- /bin/sh -c 'set zfs:zfs_initialize_value=0; sudo zpool initialize bpool {bpool PARTUUID}; sudo zpool initialize rpool {rpool Drive 1 PARTUUID} {rpool Drive 2 PARTUUID} {rpool Drive 3 (SLOG) PARTUUID} {rpool Drive 4 (SLOG) PARTUUID}; while sudo zpool status | grep "initializing" > /dev/null; do clear; sudo zpool status -Td; sleep 2; done; clear; sudo zpool status -Td; sleep 15'
... to zero the rpool free space.
Record the UUID (not PARTUUID) of the swap partition for later use:
sudo blkid
Unmount swap partition: sudo swapoff -v /dev/sda2 <== The swap partition
Zero swap partition: sudo dd if=/dev/zero of=/dev/sda2 bs=512 status=progress
Set up swap partition: sudo mkswap /dev/sda2 -U {Swap Partition UUID} <== The original UUID of the swap partition
Mount swap partition: sudo swapon -a
Zeroing the sectors on the swap drive resets its UUID. Setting the UUID as done above means you don't have to mess with your /etc/fstab file, if you've set up your swap partition to be mounted like:
/dev/disk/by-uuid/{Swap Partition UUID} none swap sw,noatime 0 0
Now boot into Zorin OS USB stick and run BackupToZip.sh.