I finally cracked it!
I was running the salt-call straight from the script and completely missed the >dev/null
Removing the pipe, I then got:
One step closer... so let's run get_root_filesystem:
python3-pyudev dpkg is installed though... but running apt reinstall -f python3-pyudev and now working:
Re-run the dpkg install and everything went fine (well... various DNS issues following the upgrade but I fixed this by forcing DNS servers through systemd-resolv - no idea what the deal is there)
So yeah... not too sure where that python package went during the upgrade!