I am running truenas community edition on an hp prodesk 400 g1 sff, with an rx 550. I am unable to headless boot without a real display, as even a dummy hdmi display doesn’t work. I tried enabling integrated graphics to see if that would help but it did not. I feel like this didn’t happen until I installed truenas because I used the hdmi dummy when I had PopOS installed and it worked.
(I am unable to include a link to the dummy hdmi, but let me know if this would be important and I can try again then)