No, it happened before:
Jan 22 11:34:40 Tower emhttpd: copy: disk1 to disk29 running
...
Jan 22 22:09:12 Tower kernel: kernel BUG at fs/buffer.c:3351!
...
Jan 23 10:23:58 Tower emhttpd: copy: disk1 to disk29 completed
If you reboot you'll just need to start over with the copy, and see if there's no crash this time.
There was a kernel crash in the middle of the copy operation, but according to the syslog it completed, not sure if the crash is related or not, try refreshing the GUI, if the rebuild option still doesn't appear I would reboot and start over.
The amount of writes reported in the GUI is basically meaningless, it can vary wildly with the device/controller used, you need to check the SSD SMART report and then again after 24H to see the actual writes.
It's what btrfs is reporting, and I bet that it's correct, but if it isn't it's not an Unraid problem, at most could be a btrfs issue, you'd need to report it for example in the btrfs mailing list or their IRC channel.
Problem with the HBA:
Jan 24 09:33:38 unRaid kernel: mpt3sas_cm0: SAS host is non-operational !!!!
Make sure it's well seated and sufficiently cooled, you can also try a different PCIe slot if available, failing that try a different HBA.
Once a device gets disable it needs to be rebuilt, just changing cables/slot won't fix anything, you can rebuild and see if the problem occurs again, if it does replace the disk.
If you don't need graphics use the x16 slot, x4 used with a PCIe 3.0 HBA still has plenty of bandwidth for 8 drives, just keep in mind that slot goes thought the DMI, so bandwidth is shared with the remaining SATA ports, etc.
Not surprisingly GUI is showing the correct usage, btrfs reports around 165GiB used (or 177GB), and that is the actual used space, like mentioned du isn't reliable with btrfs.