Great, I assume you know that completely filliping up a filesystem can be an issue, though usually not problem with btrfs if it's a WORM kind of scenario..
You cannot add devices to a xfs pool, they are singe device only, you can devices to btrfs pools, and you don't do a new config, just stop the array, add new device, start array.
Did you change the filesystem by clicking on cache on the main GUI page or by editing a config file?
Please reboot and post new diags after array start.
Parity should be OK for now, but those are low end SMR drives, and based on the number of issues I've seen with them they are probably not the most reliable Seagate drive.
Yes this is the same problem, you can try running without the plugins listed above and/or switching to ipvlan, we are still investigation, and still not clear what causes this issue.