Back to the Roku, you have a few options to get captions from your ripped media. First and easiest is to install Plex server on a computer on your network. Plex has a great user interface, and is capable of decoding image based captions and burning them into the video stream. But that requires a computer with a fair amount of power to transcode the video, especially if you're decoding 4K ripped media. Another option is a DLNA server such as Serviio. It too can decode the captions and b u r n them into the video stream, but it has the same computer power needs. Not mega powerful, as my primary media PC is "only" an i5-6400 and 8 GB of ram. But not anything really lightweight.