Totally worth it even in SDR, the pure blacks are so much better than any other form of backlighting, and even without HDR the colors are much more vibrant.
It works just fine actually, the full (non-lite) version. For how long is still a question but the Vivaldi devs are keeping manifest v2 support alive for now.
For the CPU: I have a AMD Ryzen 5 8600G in my TrueNAS Scale box, I use the integrated graphics for hardware transcoding in Jellyfin. If you don't need transcoding it's probably overkill.
For TrueNAS you'll also want a lot of RAM, as ZFS (the filesystem) gets faster with more ram by using it as a cache. My box has 32 GB, but 16 GB should also work, especially if you're going with SSD storage.
And don't go cheap on the power supply, get an efficient one from a good brand (I prefer BeQuiet).
Totally worth it even in SDR, the pure blacks are so much better than any other form of backlighting, and even without HDR the colors are much more vibrant.