Probably some of the Nazi leaders may have been opportunists to varying degrees, but to say that most of the leaders were not hardcore Nazis is not accurate. Out of the various biographies and books I've read, they're were all aligned about racial supremacy, antisemitism, and German expansionism, this is 100% crystal clear. Even private diaries from people like Goebbels that are now public, makes their commitment abundantly clear.

Another example is Albert Speer (whose biography I've also read), where he initially was an opportunists but eventually became active participants in furthering the goals and believing the "mission" of the Nazis, even though initially (and afterwards) wasn't as convinced (edit: by his own accounts, many historians disagree with this today).

Characterizing the leadership/inner-circle/leaders as merely power-seekers who didn't believe their own ideology minimizes their moral culpability and misrepresents the historical record.