I think you overrate the influence on science on German nationalism. The idea that blood relationships matter doesn't come out of science. It's a much older idea.

The specific ideas behind Nazi eugenics and "racial hygiene" derive — in part — from earlier eugenics and racial-hygiene movements in the US. See, for instance, the Indiana eugenics act of 1907 and, more pointedly, the Virginia Racial Integrity Act of 1924, which combined legislation against mixed-race marriages with compulsory sterilization of the "feebleminded".

and new ones like middle-class fear of the poor and rural

What are you talking about? Late 20st century US thought?

No, early-20th-century US thought,. I should have been more clear: by "new" I meant "new at the time", not "recent as of today". The idea that poor rural families were inbred hives of criminality, madness, and race-mixing was one of the motivations behind American eugenics of the early 20th century.

Thing is, it's true that many mental disorders are heritable. In that regard, the early eugenicists were not operating entirely on pseudoscience. But they went wrong in believing that if nations refused to use law and violence to control people's reproduction (and, ultimately, to kill the "unfit"), that society (or "the race") would degenerate.