IMO the tag serves its purpose. "Nazi Germany" is a well understood term to describe an important period in German history under the Nazis. Once you're here a bit more, you'll find this hasn't been a source of confusion (examples on History.SE where it was are welcome). What has been is people coming in with an agenda to cause confusion, those are quickly downvoted and/or deleted. This is a general problem with any controversial historical topics.
As to the proposed tags, they are not synonyms.
National-Socialism can be from any period and not specific to Germany.
historic-germany spans the entire history of Germany.
germany-during-ww2 does not cover the whole of the Nazi regime which existed prior to WW2.
Another attempt might be
germany-under-the-nazis but this implies the Nazis conquered or occupied Germany rather than being the (arguably) legitimate government. Compare, for example,
Each tag has a short and long description to avoid just this sort of confusion. Here is
nazi-germany's short description at the time of this writing.
The period of German history from 1933 - 1945, specifically regarding the rise of the National Socialist German Workers Party.
Maybe you can improve it?