wunderkind is a loanword, it's one of those cases of a German word being used but being odd in English since it's so similar.
Like kindergarten which is often speller as "garden".
You stated Endgegner was not used lightly in Germany when it was. You seemed to think Wunderwaffe was used lightly in English when it was not. And searching for Endgegner and Endlösung found our comments and a few opinions they sound similar. No evidence or claims of origin. I conclude Endgegner does not originate from Endlösung probably.
Certainly, so we have to critically look at the terms and check which have been used by the Nazis to promote their ideology. It is a call to make in each case.