Very impressive finding, @Mort1!
The issue that I had is different than yours, however. In my case, the DOM was always recognized as either sda or sdb (not sdx). I can’t confirm whether it was sda or sdb as I don’t have the NAS in front of me at the moment. Following the firmware recovery instructions, I did remove all drives from the NAS to avoid any unexpected issues. Did you also remove all drives from the NAS when you made this discovery?