Hi and welcome to the forum
I have a D8 Hybrid and did some test back in November. I was able to see the disks in Truenas when using it in single mode.
It’s been on my todo list since then to do more tests but I never get to it.
Quote from my post in an earlier thread about the D series. (Do the TerraMaster "D" series (enclosures) play nice with TrueNAS? - #2 by etorix).
I did some test initial testing with a D8 Hybrid for that scenario of secondary backup. It was on a virtualized Truenas Electric Eel under Proxmox.
If I remember correctly, I was able to see the drive serial numbers.
I got some problem with a 3x4TB raidz1 pool with some possible corruption, probably because of the usb connection. Scrub did not find any problem but the pool was unhealth and the replication stopped.
A single disk stripe pool was working better but again, very limited testing.