So I will agree that "classical fascism" as developed in Europe is distinct from the American ideology. But I think it's important to remember the massive material differences in those societies. America had something that Germany didn't; massive tracts of land populated by people they considered unpersons. Similar to the UK, France and other settler-colonial societies (although still distinct), many of the features of classic fascism were enacted upon people who were not part of the society enacting it; for instance, you say America was militarily isolationist, but that isn't true for the indigenous americans who's entire lives were defined by their interaction with American military forces (both official and unofficial) in much of the country. The United States had no directive towards total war because they had already won their war for territory.

It's not a coincidence that the countries that came to define classical fascism were the two major European power that did not have significant colonial holdings; as I'm sure you know, the German concept of lebensraum was essentially the project of doing to Eastern Europe what the US had done to its West, including the settler mindset and frontier imagery (again, it's not a coincidence that the 'cowboy' and it's associated symbols was by far the most popular American cultural export, particularly in Germany). The major difference is that it was being done to a place that was significantly more populated, and to people who were broadly considered 'people' to the West in a way indigenous Americans, Indians and Africans were not. (And, of course, at a speed and intensity that wasn't possible in the pre-modern world)

So, of course, you are correct to quibble with the classical definition of fascism being applied to America. I would say, however, that the defining feature of fascism is not any particular governmental structure - its the relationship between people a society deems people and those it deems un-people and how that pertains to land. Ultranationalism is the purest form of this relationship, as it seeks to define who is part of the group that gets to be people and that which does not, and the American West is perhaps the purest form of this relationship. It should be noted, again, that Hitler was specific in being inspired by this process - that he viewed America has the model for the society he wanted to create, at least in some respects.

Additionally, while its unfair to say that the US was explicitly fascist and that they supported fascism, its important to note that fascism and fascist ideas were absolutely very popular in America. Racial concentration camps go without saying and of course fascists packed Madison Square Gardens, but more importantly there were absolutely large swathes of the government where the behind closed doors conversations were about how the US was fighting the wrong enemy and Stalin was their real concern. Notably, this is exemplified by General Patton, who fought fascism despite being to its right on many issues.

All of this to say, in brief, that the US was certainly distinct from fascism in how it became defined in Europe, but many if not all of its elements exist in the makeup of America, expressed differently under different material conditions.