Free space and the space taken up by metadata and snapshots can take a little attention to figure out
https://ohthehugemanatee.org/blog/2019/02/11/btrfs-out-of-sp...
https://ohthehugemanatee.org/blog/2019/02/11/btrfs-out-of-sp...
If I could mount the failed btrfs filesystem read-write for an appreciable length of time, I could try the balance operation.
I'll need to set up a cron job on the other systems to run balance on a regular basis alongside scrub (obviously not at the exact same time).