At boot there were only 4 devices found from the pool, devid 2 was already missing:
Mar 26 15:25:41 BigServer kernel: BTRFS: device fsid 6256fb88-756b-4111-8cb1-eb5ebd47f76b devid 3 transid 2574301 /dev/sdj1 scanned by udevd (758)
Mar 26 15:25:41 BigServer kernel: BTRFS: device fsid 6256fb88-756b-4111-8cb1-eb5ebd47f76b devid 5 transid 2574301 /dev/sdd1 scanned by udevd (758)
Mar 26 15:25:41 BigServer kernel: BTRFS: device fsid 6256fb88-756b-4111-8cb1-eb5ebd47f76b devid 4 transid 2574301 /dev/sdh1 scanned by udevd (745)
Mar 26 15:25:41 BigServer kernel: BTRFS: device fsid 6256fb88-756b-4111-8cb1-eb5ebd47f76b devid 1 transid 2574301 /dev/sdl1 scanned by udevd (769)
This was devid 2, it was assigned but there isn't a valid btyrfs filesystem there, can't see why since it was already like that at boot
Apr 5 20:52:39 BigServer emhttpd: import 31 cache device: (sdk) MTFDDAK256MAM-1K12_14170C18C9B8
After that you unassigned a different device and started the array, this resulted in Unraid wiping another device:
Apr 7 19:15:14 BigServer root: /dev/sdd1: 8 bytes were erased at offset 0x00010040 (btrfs): 5f 42 48 52 66 53 5f 4d
So now there are two missing pool devices, you can try this:
btrfs-select-super -s 1 /dev/sdX1
Do this for sdk and sdd, if you rebooted since check that the device identifiers are still the same, if the command is successful for both, unassign all the pool devices, start the array so that Unraid can "forget" the pol config, stop array, re-assign all 5 pool devices, start array and post new diags.