Cache filesystem crashed during the balance because it ran out of space, problem is that now it's read only so you cant delete anything, reboot and post new diags but you'll likely will need to mount manually with the skip_balance option or it will probably go read only again, alternatively and if you have backups just re-format the pool.
You can do a correcting parity check, but if parity is way out of sync it will be faster to re-sync it, to do that stop array, unassign parity, start/stop array, re-assign parity, start array to begin parity sync.
We can 't see why the disks got disable because of the reboots but they look healthy, and two disks getting disable at the same time is seldom a device problem, since the emulate disks are mounting and assuming contents look correct you can rebuild on top:
https://wiki.unraid.net/Manual/Storage_Management#Rebuilding_a_drive_onto_itself
More of an annoyance, it only happens when converting pools to RAID5/6, GUI flickers several times before starting the balance, first noticed it in one of my main servers, confirmed in safe mode in a different server using multiple browsers, see attached video:
2022-05-16 17-10-39.mkv
Unraid doesn't stripe data, so no file can be larger than a single disk, not sure how to fix the VM problem, likely you'll need to use some OS specific recovery tools.
You can try this mentioned in the link:
If all else fails ask for help on the btrfs mailing list or #btrfs on libera.chat
Other than that can't help more with that error, since apparently all superblocks are damaged.