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).
+1 for Sumatra as a viewer, it's super fast, even with large documents.
Okular has support for filling forms, but I'm not sure if it has more advanced editing features.
A non-adobe, but closed-source alternative is Foxit. It's Chinese, so not EU, but also not US (: