huh I always thought Nazi Germany was not imperialistic but instead driven towards a hegomonic ethno state.
For me the difference of empire and national state was mostly an aspect of a realm with multiple cutlures, with one culture dominant.
So Britain was an empire, when it controlled multiple territories that other ethnic groups lived and dominated them. Same with Ottomans, Austria, France and so on.
When France lost Algiers, and only had Frenchmen in their territory, they weren't an empire anymore.
On the other hand, Switzerland isn't an Empire, since multiple ethnic groups live peacefully side by side without one dominant ethnic group.
Japan very much was an Empire, with Korea and the korean people under their control, Italy aswell with Libya and Ethopia, but Germany first tried to unify germanic people, and later expand its territory, without wanting the former people of the land still living there.