After a bit of a slow start trying to put `-one-file-system` early in the command chain, I hit pay-dirt landing that -one-file-system argument at the end of the command did the trick:
I have a fair few mount points in this heavily customised, heavily used Kubuntu system. /home is mounted, as is /tmp oooh I spotted that dirty dirty snap mount mess (not a fan of snap, here is my thoughts on snap).
nice sudo mksquashfs / /home/toms-kubuntu-daily-driver-kubuntu.sqsh -one-file-system
Parallel mksquashfs: Using 16 processors
Creating 4.0 filesystem on /home/toms-kubuntu-daily-driver-kubuntu.sqsh, block size 131072.
//boot/efi is on a different filesystem, creating empty directory
//dev is on a different filesystem, creating empty directory
//home is on a different filesystem, creating empty directory
//proc is on a different filesystem, creating empty directory
//run is on a different filesystem, creating empty directory
//sys is on a different filesystem, creating empty directory
//media/tom/4tb_cifs is on a different filesystem, creating empty directory
//media/tom/media.soul is on a different filesystem, creating empty directory
//mnt/3tb_ext4 is on a different filesystem, creating empty directory
//mnt/tape is on a different filesystem, creating empty directory
//mnt/3tb_xfs is on a different filesystem, creating empty directory
//mnt/1music is on a different filesystem, creating empty directory
//mnt/data is on a different filesystem, creating empty directory
[| ] 0/42543 0%
//snap/bare/5 is on a different filesystem, creating empty directory
//snap/core22/1564 is on a different filesystem, creating empty directory
//snap/core22/1586 is on a different filesystem, creating empty directory
//snap/firmware-updater/127 is on a different filesystem, creating empty directory
//snap/gnome-42-2204/176 is on a different filesystem, creating empty directory
//snap/gtk-common-themes/1535 is on a different filesystem, creating empty directory
//snap/snapd/21465 is on a different filesystem, creating empty directory
//snap/snapd/21759 is on a different filesystem, creating empty directory
//snap/thunderbird/497 is on a different filesystem, creating empty directory
//snap/thunderbird/507 is on a different filesystem, creating empty directory
//snap/krita/104 is on a different filesystem, creating empty directory
//snap/kf5-5-113-qt-5-15-11-core22/1 is on a different filesystem, creating empty directory
//snap/djpdf/134 is on a different filesystem, creating empty directory
//snap/djpdf/135 is on a different filesystem, creating empty directory
//snap/core18/2829 is on a different filesystem, creating empty directory
//snap/verminest/1 is on a different filesystem, creating empty directory
//snap/gnome-3-28-1804/198 is on a different filesystem, creating empty directory
//snap/wine-platform-5-stable/18 is on a different filesystem, creating empty directory
//snap/wine-platform-runtime/388 is on a different filesystem, creating empty directory
//snap/wine-platform-runtime/389 is on a different filesystem, creating empty directory
//snap/flutter-folio/3 is on a different filesystem, creating empty directory
//tmp is on a different filesystem, creating empty directory
[\ ] 0/495395 0%
//var/lib/portables is on a different filesystem, creating empty directory
//var/lib/machines is on a different filesystem, creating empty directory
[| ] 3884/1385606 0%
Unrecognised xattr prefix system.posix_acl_access
Unrecognised xattr prefix system.posix_acl_access
