Thank you for the explanation.
One way of creating the distro is by making a meta package, which would pull in all that’s needed, add ubiquity to that, change whatever that’s necessary and make a squashfs file off it, create standard additions for the live iso, that is, what you’d see, when you unarchive the ubuntu.iso you download. This is what the remastersys script does, but from an installed system.
You can find an inspiration from here too, https://github.com/peppermintos/peppermint-iso-builder/blob/master/builder.sh and https://github.com/peppermintos
If you look at Unity7N.iso, all I did was to unarchive the then downloaded Ubuntu 17.10 iso, took away the filesystem.squashfs to a separate edit folder, unsquashed it, went into chroot and installed unity-session, lightdm, and uninstalled gnome-shell, gdm etc, and cleaned it up, squashed it back, put it back in casper and create the iso. That’s in a nutshell.
You can find the complete information here, https://help.ubuntu.com/community/LiveCDCustomization
That Unity7N.iso can be upgraded to today’s 17.10 or change repos to bionic and upgraded to up to date18.04. I did that few days ago and everything worked well.