1. The mini.iso can not be customized in Cubic, because it is not a desktop environment. I am assuming this bug report is not specifically for the mini iso.
2. You can not use a Wayland session to run Cubic, because Wayalnd does not allow users to run graphical applications (like Cubic) with sudo privileges. (I don't think you are doing this, because your $XDG_SESSION_TYPE shows "X11", but I thought I'd just mention it, since the original bug report mentions a permissions issue).
Looking at the original bug report, I see a permission issues with /run/user/ 1000/dconf/ user.
Here are the permissions and ownerships on my "clean" install in VirtualBox...
drwxr-xr-x root:root /run 1000/dconf 1000/dconf/ user
drwxr-xr-x root:root /run/user
drwx------ <you>:<you> /run/user/1000
drwx------ <you>:<you> /run/user/
-rw------- <you>:<you> /run/user/
("<you>" should be your user id)
Remember:
1. The mini.iso can not be customized in Cubic, because it is not a desktop environment. I am assuming this bug report is not specifically for the mini iso.
2. You can not use a Wayland session to run Cubic, because Wayalnd does not allow users to run graphical applications (like Cubic) with sudo privileges. (I don't think you are doing this, because your $XDG_SESSION_TYPE shows "X11", but I thought I'd just mention it, since the original bug report mentions a permissions issue).