Honestly wish I was more help, new to this whole hexos/truenas thing... it is 100% working, plex shows HW in the dashboard when transcoding and CPU usage doesn't jump with 6 transcoded 4k streams going.
As to what I did... I tried installing plex through Hexos, which made the folders and stuff for me, but transcoding didn't work despite showing the GPU, ended up uninstalling plex, and reinstalling via truenas using the default save paths that Hexos did, that also didn't work. I then uninstalled again and installed using truenas default install paths, and just adding the paths for my movie storage. Then the transcoding started working fine. I'm sure it's some permission issue in the background somewhere, but I couldn't figure it out but this worked for me. Your mileage may very lol