I know of at least one other person working on the same issue. Maybe you guys can collaborate:
Let me clarify the headless method you might try. IF the issue is that the non-free image of StartOS is missing the necessary proprietary graphics drivers needed, you can bypass it by not connecting a monitor. First you need to make sure the bios options are set to boot the USB drive as primary, and the internal drive as the second. You’ll need to also look for a “stop on keyboard error” option, and disable it. Once you’ve applied these options, and saved. Turn off the machine. To test, leave the monitor connected, but disconnect the mouse, and keyboard. If you have things right it will boot into StartOS, but give you the same error. Next, turn off the machine. Disconnect the monitor. Make sure the S13 is plugged in, via ethernet to the same router or switch as one of your other machines. Avoid using wireless on the other machine if you can. Turn on the S13, and wait for about 5 min. You might hear a series of beeps. If so, the final sound that means the server is ready, some people describe as a Mario getting a coin sound. If you hear nothing, that’s fine, just wait more than 5 miin. Then on the other machine, that’s connected to the same router or switch, open a browser and enter start.local into the address bar at the top, and press enter on the keyboard. If that works, then you are here:
https://docs.start9.com/0.3.5.x/user-manual/initial-setup/initial-setup-start-fresh