It's probably much too late for this to be helpful, but the issue with not all flash drives being compatible is because that version of uboot would probe for US

It's probably much too late for this to be helpful, but the issue with not all flash drives being compatible is because that version of uboot would probe for USB devices very early on and it doesn't probe again if it doesn't find anything. Changing bootdelay won't fix that because that happens afterwards. It can boot from practically any flash drive if you prepend "usb reset;" to bootcmd and then do a saveenv. Be careful when doing this, all of that configuration isn't stored on the flash drive obviously so if you aren't comfortable with uboot you can easily brick the router. Just do a printenv first and make sure to copy down the old bootcmd just in case you need it to recover.