A few things.

Depending on how old your FFMPEG is , you may actually be decoding the LOSSY DTS core part of DTSHD. DTSHD decoding via dcadec was only very recently added to FFMPEG (mid 2015).

MakeMKV is updated regularly, so as long as you have a recent version of MakeMKV AND the makers of MakeMKV have decided to use a recent FFMPEG, you MIGHT be ok.

I would assume that MakeMKV/FFMPEG decodes to the bitdepth that the lossless part of that particular DTSHD stream specifies. DTSHD MA can be 16, 24, or any number of bits in between.

I would also argue that 16 bit is OK for listening, and even for editing, and dither is bad. It’s:

Potentially harmful
Mostly useless
Self-defeating

For converting to 16bits, I highly reccomend truncating the bits (no dither).

At sensible volumes, you won’t need dither until you get to approx. 12bits.