OK so I started again, and this time it seemed to go mostly fine. In case any other idiot does this, here's how I got it running
apt-get remove --autoremove openmediavault
Then instructions from https://openmediavault.readthe…stallation/on_debian.html
Then omv-engined would not auto-start so..
systemctl unmask openmediavault-engined
OMV-Extras would not re-install until I did a
apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 7AA630A1EDEE7D73
And apart from the fact that it seems to now cache my login in the browser (if i reboot it, it does not ask me to re-auth) it seems to all work fine.