Also have a 6800 Pro running TrueNAS for a week or so now.
Completely disabled the UGOS drive through the boot menu (just to prevent myself from accidentally overwriting it or something silly 
) and connected an NVME drive in an external enclosure through one of the USB slots on which TrueNAS has been installed.
It’s all been working pretty well, even the LEDs using the binary method described here, with a slight update to the example script to make it work with the 6800 Pro (which I shared in a comment on that gist).
The only real issue I encountered was the ZFS cache eventually completely filling up, causing the entire system to become unresponsive. It doesn’t seem to be specific to the UGreen devices, as I found an issue describing the issue here, including a work-around (echo n >/sys/kernel/mm/lru_gen/enabled which I’ve added as a POSTINIT script to TrueNAS, which seems to have resolved it).