The parity swap procedure should work, so I wonder if you got all the steps right.
i do not see much point in adding the 8TB as parity with a bad drive present. I would be tempted to remove it, start the array to commit its removal. At this point you should see your bad disk as not present and it’s contents being emulated. You can now try the parity swap procedure again and come back here immediately with new diagnostics if it does not work so we get a chance to see what happened.