If I remember right gparted with btrfs-tools doesn't create the same version of btrfs as SailfishOS does. I can't verify it now though (no phone yet). I use gparted too.

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On topic, while I researched about btrfs, wouldn't this be possible:
First change the mount-sd.sh as said here

Then create btrfs subvolume(s), for example music:
btrfs subvolume create /user/100000/media/sdcard/Music

Copy everything from /home/nemo/Music to your newly created subvolume, and then mount it to home:
mount -o subvol=Music /dev/mmcblk1 /home/nemo/Music/

This way I could copy all my music from computer to my sdcard via MTP without symlinks, just copy/paste to Music folder in Jolla. What would be the downsides of this? The subvolumes should mount automatically too I think, since the mainvol is -thanks to the modified mount-sd.sh. What's better here than symlinking? Well at least for me the symlinks to sdcard didn't work after phone reboots -even after mounting my microsd manually before trying- and I had to create the symlinks again every time, but then again I didn't test this after the changes to mount-sd.sh script, so it may not be a problem.

This may be heavily just a personal preference: do it the way you like.

I'll experiment with this when I get my phone.

And oh, my problem with my btrfs-microsd wasn't due to problems with btrfs filesystem, the problems continued even after formatting to ext2. I begin to suspect either my microsd is faulty, or my Ubuntu has problems (which it had, I had to reinstall it just a few days ago). We'll see.