I have converted high bit rate video files (20mbps to 40mbps) to mkv (matroska) format with HEVC encoding. I have used both Handbrake and TmpGenc. I usually use a target bitrate between 10 and 15mpps (depending on the apparent "sharpness" of the source material).
I personally cannot perceive any visible degradation, although there must be some loss of detail.
Once the video is compressed, often the DTSHD audio track is as large as the video component!
I have experimented with converting the DTS audio track to FLAC. The matroska (mkv) file container can handle HEVC (H.265) video with either a DTS or FLAC audio (as well as many others).
Occasionally I find that the FLAC audio stream is significantly smaller than the DTS. On other occasions they are of similar size.
On a small number of files I found I had audio sync issues using the FLAC audio. When this happened, I just retained the DTS. Also, many devices seem unable to play DTS audio, so a conversion to FLAC can help here.
Keep in mind that the HEVC video decoder requires a fairly powerful computer. Older computers and particularly older phones and tablets can struggle with HEVC.
I had the tools to do this as I produce small videos using HEVC and so I have the tools required.
As a starting point:
Handbrake (free) for video encoding as HEVC
MediaCenter for the DTS to FLAC conversion
MKVToolNix (free) for matroska packaging and for extraction of video and audio from .m2ts
I am not making any recommendations, just reporting my experience.
Dennis MF