Xbox might be an alternative. See the “Plex, 4k, …” thread. Still early, but looks promising.
The new Plex HTPC client will be an option at some point (it is the replacement for Plex Media Player). It passes the audio. Not sure if HDR is working correctly yet. It is still in development. You can try the preview release if desired. Runs on Windows & Mac, and Linux some day.
The Shield includes a SMB client as part of the Android TV OS. You can keep the media on the NAS when running PMS on the Shield. Just mount the shared folders on the Shield that hold your media.
The Plex data folder, with metadata, thumbnails, etc, stays local to the Shield. It can be moved to an external USB drive. It cannot be moved to a network share (not a good idea anyway).
