If you are on a recent enough Sailfish version, you can try playing with the btrfs-balancer command, e.g. "btrfs-balancer balance" (as root...).

Otherwise, look up the older balancing instructions in jollatogether.

Please note that btrfs is worse than ext4 in almost any benchmark, and I would even say has a more stupid design, save for precisely the features you don't want (subvolumes, snapshotting, CoW). _Specially_ the completely broken version that is shipped on the Jolla kernel. Therefore, maybe you want a ext4 formatted SD card...