When Nikon ceased support for CNX2 and their new software consisted of CNX-D in it's initial versions, I also looked at the option to convert NEF files to DNG and use Lr.
Hated Lr and it's impossible to work around quirks, so abandoned the conversion to DNG format idea.
Looked closer into it, and found that many DNG conversion programs obfuscate some of (maybe non essential) maker note metadata, among other stuff in the original raw file.
if you want to preserve ALL the data(basically metadata, not image data) in the NEF file, you should find an option to preserve the raw file into the new DNG file. This way the NEF file isn't then mangled.
Downside tho is that the new DNG file will be larger in size to the original NEF file .. so no point if space saving is the goal.
After testing, I found it wasn't worth the effort.
Being curious tho, I downloaded sample Pentax natively saved DNG files, and compared to PEF files .. and at least there was no difference in the metadata.
if you use Nikon's software in any way, and want to save some space, lossless NEF compression can save some space but it depends on the actual image as to how much it saves.
Opening this losslessly compressed NEF in Nikon's software can allow you to then decompress the image again with an increase in resultant file size.
Then if you want again, you then use the Nikon software to save the NEF with compression again, lossless or lossy(or none).
Again, the amount of storage space was too variable to give an accurate prediction, but on a 90Mb D800E file it could be anywhere between 10-20Mb .. sometimes 30-40Mb(eg. an image of the moon) on a 75Mb uncompressed NEF(ie. half)
problem is that CNX2 is no longer supported and one will become totally unusable on a modern operating system.
No idea what CNX-D does with respect to saving NEF files in this manner, never looked at it.