A "Server/Renderer" has to do with DLNA interfacing using more than one device. For example, you could have a computer with JRiver software. JRiver can act as a "DLNA Server", which means it is a network based source for storing and streaming media content. A "Renderer" is a target device that will absorb and "play" media content. For example, a "Renderer" could be a bluray player on your network. Using Jriver, you could target the bluray player to "render" music files (essentially "play" the music files).
As far as the Sonica is concerned, it is not entirely clear from the documentation, but the way that Oppo words it, I would interpret this as the Sonica app on the android/iPhone/iPad will connect to Tidal and stream down Tidal music. Then Sonica app will then send that audio directly to the Sonica DAC. In this case, the Sonica app sort of acts as a "DLNA Renderer", but it still has to pass it on to the DAC (so it’s sort of like a "renderer" for another "renderer" (the Tidal music service would be like the originating "DLNA Server", but it uses special Tidal interface instead).
I think the reason people have DLNA Servers is that they have a very high dollar DAC that they want to play the audio with (like many thousands of dollars). Also, these server renderers can put large focus on digital circuits to reduce jitter and make clock pulses with exact timing. This is good, but it does cost money.
If you are happy with the Sonica in your budget, I would say to get it. The Oppo software is excellent and you can spend money on Sonica modifications, which will further improve things! For $800, you won’t be able to get a server/DAC combination that will come close to the Sonica.