I ran into the same read only issue with btrfs and raid5/6
After some poking around I stumbled on this web page
Unable to import BTRFS pool into Rockstor 4.0.8-0 which was a similar but slightly different issue
To distill it down here’s what I did which solved the to problem
echo “options btrfs allow_unsupported=1” > /etc/modprobe.d/01-btrfs.conf
mkinitrd
reboot
Good luck
Pete