I misremembered that Gen3 hardware had a spare PCIe slot, my bad.
You can either forego NVMe slots (which looks like an add-on card on [0]) and get the slot, or use one of the USB4 interfaces. OTOH, it has 2x10GbE on board, you can just media-convert it.
That seems like a lot of effort - is there no ability to boot a custom thumb drive that loads something like an SSH terminal, or dummy display for VNC?
The problem is not getting TrueNAS on a disk. You can do it externally, but you need to disable the on board flash storage and change the boot order from the BIOS.
That box is "just" an I/O optimized PC which can boot without a GPU.
Older hardware with Intel processors have an iGPU on board. You can use the HDMI output on these directly.
You can either forego NVMe slots (which looks like an add-on card on [0]) and get the slot, or use one of the USB4 interfaces. OTOH, it has 2x10GbE on board, you can just media-convert it.
[0]: https://www.youtube.com/watch?v=wWgc8W-hIWM