This link helps your btrfs-card to achieve the basic functionality and automount-behaviour.

For more advanced functions, and to achieve similar results like in Leinad's OP, I recommend you to look at J4ZZ's post
Note: You need a script editing application to edit mount-sd.sh, you might want to hit nieldk with a message if you need one I noticed there's vi pre-installed in Jolla already

EDIT: Well, you chose the ext4 path, which may not have been unwise