Since I'm a new moderator here I figured I should actually install Debian, but I decided to do it with a twist.

I got the idea from here: https://codeberg.org/Calamares/calamares/issues/1580

It's actually not too hard to install multiple Linux distros sharing the same Btrfs partition, which means you don't have to preallocate space.

Default Calamares mount.conf here https://codeberg.org/Calamares/calamares/src/branch/calamares/src/modules/mount/mount.conf#L50-L57

My 40_custom file:

#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.
menuentry "Kubuntu" --class kubuntu --class ubuntu --class os {
 insmod part_gpt
 insmod btrfs
 search --no-floppy --fs-uuid --set=root c9710fb0-4ebb-4335-a58a-4176d894dcf9
 linux /@/boot/vmlinuz root=UUID=c9710fb0-4ebb-4335-a58a-4176d894dcf9 ro rootflags=subvol=@ quiet splash
 initrd /@/boot/initrd.img
}

I was disappointed that os-prober didn't work.