Maybe I’m failing to understand something, but wouldn’t you just continue to use the TrueNAS host for storage and simply use the proposed intel host for Plex transcoding and nothing else? You could load proxmox or Ubuntu server and run plex server in a VM/docker/LXC. Then you can use the beefy Threadrippers for the rest of your VMs that need lots of CPU & RAM.
How many plex transcodes do you ever see at once? More than 20? More than 30? I’m not saying you shouldn’t go with whatever route you want to - but if you’re looking for the most cost efficient method than I still fail to understand how threadrippers and quadros are most cost efficient.