Thanks for going into this!
My immediate suggestion would be to separate the part that does the set up of LXD and the tear-down.
It is a more generic task that of configuring LXD, so such a script would be useful elsewhere as well.
In addition, you can use a preseed file when you set up LXD and also a profile for the guiapps containers.
See https://blog.simos.info/a-closer-look-at-chrome-os-using-lxd-to-run-linux-gui-apps-project-crostini/ and specifically the link with the preseed content that they use for the Chromebooks.
In addition, when you set up ZFS or BTRFS for the storage backend (instead of dir) you get much faster container management. That would make it easy when you want to extend with some GUI program like VirtualBox clone.