Thanks for replying to my query, James … appreciated.
By native did you mean macOS … Apple TV 4th and 5th Gen supports it?
I don’t think QuickTime Player can transcode on the fly … but it will play HEVC from it’s own container format?
For 1080p (macOS) it is … just change the MKV container for M4v (it only takes seconds). Airplay with QuickTime.