I dont quite understand this point of view. Dont get me wrong, I understand the idea of a safety buffer from the max ratings. But ALL the pieces are similar max ratings.
All the parts from truck, to hitch, to trailer are tested to the same type of standards (SAE).
Why do you trust to max out the 20k GVWR rating that was arrived at by the same standards, but not the hitch? If you say that you would stay away from the 20k GVWR for safety as well, then that still doesn't make the point of view make sense, since you will be staying away from the 20k hitch rating by the same margin!
Maybe im missing something, so let me know.