You could simply throw more hardware at it and for 1 client you would be ok with the build you have spec’d out.
However, you should probably try to understand why the Roku(Ultra/Premiere?) is asking the server to transcode the movie and the most likely culprit is Audio. Newer media comes with Lossless Audio as it’s default track and you need to select the lossy track or make sure you only save the lossy track.
