Bumped because there are possible problems with my setup. If anyone uses it and experiences booting problems, remove your sdcard while your phone is off and remove all subvolume mount-commands from your mount-sd.sh script. Change your default btrfs mounting point to 0 (with btrfs subvolume set-default) and you can use your microsd like normal microsds.

By the way, I noted strange behaviour with my music files in my microsd with my setup: when Jolla's default media player played any track, it changed it's owner group to 'privileged'. This happens with file permissions set to 775 and owners set to nemo:nemo (so they are identical with content in home:nemo).