Even with the meta-package, it’d be better to install without any earlier DE in the Ubuntu (derivative) iso downloaded. That is, as there is no 18.04 mini.iso available. You can do all that through Cubic in a chroot environment. Personally, I’d choose to do that manually, to see what’s happening, rather than depending on a GUI app.
The idea, (though old, but still good) can be found here. (1) https://help.ubuntu.com/community/MakeALiveCD/DVD/BootableFlashFromHarddiskInstall (You have to do a bit of re-arranging.) From an already installed distro.
(2) https://forums.linuxmint.com/viewtopic.php?f=42&t=107350 - From the live iso. You can find it also somewhere in Ubuntu wiki and in Ubuntu Forums.
From an installed system, you can use (1) to create the squashfs file, and (2) create the live iso. Additionally, you have to isohybrid the iso for usb sticks.
(All this thanks to fragadelic!)