Thanks for the clarification! I was sloppy in my interpretation of direct streaming, and I wasn’t aware of the limitations (Plex having to use HLS when transcoding audio, and HLS not supporting HEVC).
Then one workaround might be to use (for example) ffmpeg to transform the file relatively “cheaply”, by copying the video track while transcoding DTS into AAC. That should result in a file where both video and audio are in a supported format, so that Plex can direct play or direct stream it.
With a somewhat more complex call to ffmpeg it should be possible to both copy the audio and transcode it into AAC. Then Plex might be able to extract the supported streams in direct stream mode, while the DTS track remains available for the future?