The Nazis did not create the concept of the "First, Second, and Third Reichs" but they did popularize it in their own and our times.

The concept of the First-Third Reichs comes from the German author Arthur Moeller van den Bruck in his 1923 book "Das Dritte Reich" The Third Reich.

In this book, Bruck lays out the failures of the first and second Reichs, the Holy Roman Empire and Imperial Germany. The failure to centralize into a cohesive state, and the failure to unite the entire German people, respectively.

Then Bruck lays out his concept for the Third Reich, however this was NOT a political manifesto for a real German State but an idealized perfected Greater Germany, a moral concept.

The Nazi party loved this book and looked to use its concepts for propaganda purposes. The use of "The Third Reich" in association with Nazi Germany was a deliberate attempt to enforce that it was the idealized and moral version of Greater Germany made manifest.

Arthur Moeller van den Bruck committed suicide in 1925 so avoided the main rise of the Nazis but did comment on Hitler's failed Beer Hall Putsch in that he said Hitler was devoted to Germany but lacked an intellectual foundation for the Nazi Party, and was in general critical of Hitler.

Now, you'll have noticed that I regularly have typed Reich and not translated that to "Empire" like how is usually done.

Yes, 'Reich' can mean 'empire' but setting aside that even the definition of 'empire' isn't as set in stone as many think, Reich is more complicated than that.

The cognate of Reich is the English 'Realm' which does also have a monarchist bent but also means a domain or sphere of influence more than a mere "kingdom."

The same is with Reich, it's an empire, it's a culture, a people beyond political bounds.

I'll leave if Hitler had an plans for his invented "Führer" title, which means "Leader" beyond his own time in power to others more knowledgeable on that specific topic.