If you have room in the Optiplex for another drive I would definitely get a second spinning drive for storing your media. Truenas will complain about you only having 1 drive for your media. The nice thing about having a pair of drives in a mirror is that is makes it very easy to upgrade in the future. I was running a pair of 6tb drives, until my brother gave me the 10tb ones. I just shut the system down and swapped one of the 6tb for a 10tb and booted it back up. It detected that the mirror was broken and I just told it to use the 10tb drive to fix it. A few hours later after it was done, I did it again with the other drive and then told it to expand the storage to 10tb.

If you want to get a 4tb, you could mirror that to the 1tb, (and you would be limited to 1tb) but you would have redundancy and it would be easy to swap the 1tb for a 4 in the future.

If I were you I would do what I did with proxmox. Install it on the ssd. Create a virtual machine for the Nas with a smaller vitrual disk from the 512gb ssd, 60Gb should be fine, and pass it your graphics card for the media server and the sata drives for your Nas storage and media storage. Then you have your Nas that you can use as a media server and you can also easily spin up new virtual machines to play with or test with.

I have another vm that I use just for docker containers.