Update, following the discovery of lightglitch in this thread, Import Pools and Shares after install
I have removed the @transaction.atomic flag.
This seems to have bypassed the first error, but the second is still persisting. The FS is mounting, but there appears to be an issue mounting the sub folders.
After checking dmesg I found this.
[ 559.444044] BTRFS info (device sdk): disk space caching is enabled
[ 559.447776] btrfs: cannot remount RW, RAID56 is supported read-only, load module with allow_unsupported=1
This appears to be related to the use of RAID6 in BTRFS, which is marked as unsupported and is therefor disabled by default.
Will update after enabling.
UPDATE
Pool auto mounted after making change to allow unsupported modes. Shares appear to have imported as well, however they are all marked as unmounted.
Link to doc for enabling unsupported modes.
TLDR:
echo 1 > /sys/module/btrfs/parameters/allow_unsupported