Where exactly are you having problems? Have you added the device in TrueNAS? It’s important that the TYPE is set to: USB Passthrough Device and the Controller Type to qemu-xhci, and then select your device—just like in my screenshot. Afterwards, restart your VM and look under Settings, System, Hardware, All Hardware for your “Sonoff” and copy the device path. For me, it was /dev/ttyUSB0; for you, it might be /dev/ttyACM0 or something similar. Copy that. Now install Zigbee2mqtt. My MQTT broker runs in Home Assistant as an addon, so you don’t need to configure anything else in Zigbee2mqtt except the SERIAL PATH as shown in my screenshot. I tried out the “Adapters” here; for me, it was “zstack”. If you don’t enter an adapter, you’ll see adapter options displayed in red text. ( zstack , deconz, ember, zigate)
It is absolutely essential that you use a USB extension cable. For some reason, it only works for me when I use one.