Headless setup on old SMC server

So if I have a monitor attached when the system is powered up, I can get video pretty much any time afterwards.
If I do NOT have a monitor attached on powerup, I do not get any video out the port until a reboot.

Is this a hardware thing, or is this a TrueNAS thing?
(I don’t recall if it worked before… which is why I’m asking here)

With no information about your hardware or your version of TrueNAS, it’s hard to say, but it sounds very much like a hardware thing. Depending on the the type of port for the monitor, you may be able to find a “dummy plug” like this one for HDMI:

That would fool your video adapter into thinking there was a monitor attached even when there wasn’t.