It is somewhat ironic that in the same evening when finally modified the mount-sd.sh and added symlinks, Jolla's Ohijärvi update made these obsolete.

I removed sdcard, removed symlinks. Started media player and it could not find any music, as expected. I shut down the media player, inserted my 64 GB btrfs-formatted sdcard, started media player again and presto, my music files are all there and working.

Memory card support is now way better as stated in change log. There is still room for improvement, maybe with next update...