I don’t know what your experience is with conservation. I’ve only done it once — with PCGS, not NGC — with an Indian quarter eagle. It came out somewhat OK, but was a huge pain in the butt for me and someone else.
His post was spot on, no one likes to hear negative critique about coins they love but in this instance people are trying to save you money. You have tons of awesome toners, this one just isn't one of them. It would details 100 out of 100 times at NGC, IMO.
Restoring the 2004-D Cent is like trying to restore a half melted snowball; it can’t be done. There is a reason why zinc was not extensively used in coinage until the modern era. The metal is too reactive in the environment. This leads to another question and piece of advice. Why are you thinking about wasting money on restoring copper and zinc? The chances of a good result are slim and none. With so many collectors so obsessed with “originality” these days, even restoration efforts on silver and gold are suspect. The trouble is some collectors complain if a silver coin has been dipped. Yet they won’t buy it if it’s ugly. With some people, you can’t win.