Sun Yat-Sen was basically a socialist, and had strong communist sympathies, though he was not completely on board with it. The KMT was a broad coalition of interests in Chinese society that sought to have China rise up and overcome western imperialism. It included the capitalist and feudal landlord classes, who had an interest in seeing China become a great and wealthy power and defeating imperialism, but not necessarily in disrupting the traditional hierarchies of Chinese society too much. This is why that class sat in an uneasy coalition with the Communist party and later sought to have it destroyed after Sun died and Chiang Kai-Shek took over. However, every constituency in the KMT knew that they needed to use state power and economic planning to raise China out of poverty and resist Western imperialism- methods that might be considered socialist.

My impression was that the KMT were essentially like the nationalist in German, Japan, or modern USA (they very well may have been, I suppose).

So, yes, they were nationalists, but there's a big difference between how nationalism functions in Imperialist countries verses how it functions in countries that have been exploited and colonized- in the latter case, nationalism can be a potent force for liberation. In that sense, Sun Yat-Sen and even Chiang Kai-Shek are more comparable to figures like Gemal Abdel Nasser, or Juan Peron than they are to Hitler. They were nationalists trying to overcome the contradictions of a country that had been exploited by imperialism, and they saw the necessity of socialistic methods to accomplish this. This is because economic nationalism doesn't put profits at the center of a country's economy, and a colonized country doesn't have the option of generating superprofits at the expense of a colony.

And I'm including Chiang in this category, despite the fact that he was a monstrous anti-communist fanatic, initially feckless in the face of Japanese invasion, and wound up being an imperialist puppet who was totally reliant on American support. When he actually had the opportunity to see his vision of nation-building fully carried out in Taiwan, he didn't just allow the Chinese capitalist class to pursue profits at the expense of the nation, but rather utilized their position to generate wealth for Taiwan, and undertook a very effective land reform program.

Something to remember as well, the Soviets in the 1920s were fighting a war on basically all sides against many enemies, both Russian and foreign. Backing the KMT was seen to be a pragmatic way to relieve pressure in the far East from Japan, as the KMT under Sun Yat Sen were a relatively large anti-imperial party compared to the Communists, who were at this time a small but growing faction in the left wing of the KMT.

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KMT went through a lot of changes after the Stalinbux dried up lol, and only at that point started to collab with Germany etc.

If you read Fannon's Wretched of the Earth, what everyone should read after Settlers, also Lenin's National Question both go through the reasoning why.

A small tldr is colonized have to feel a connection to their homeland as a precondition to any sort of successful movement to wrest control of themselves, their lands from colonial imperialist powers, this would be broad nationalism the precursor to internationalism, not to be confused with narrow chauvinistic nationalism, the fasc type we have in the US. You have to move out of this stage and form a socialism movement quick, or you'll get stuck in it and bad shit will happen.

Pretty much every 'third world' country outside the imperial core that has been successful in any form in the past with socialism has gone through that broad national movement with a national bourgeoisie. At first, the national bourgeois allows a colonial nation breathing room from the imperialists for nation building that allows the colonized to explore their own interests, unity, and all that. However, once this is achieved the people need to quickly put down the national bourgeois like the rabid dogs they are cuz if you don't they'll split your nation up, sell everyone and their dad out to the imperialists and turn full murderous fasc (this has happened in LatAm and Africa for failures), and turn to a socialist movement. Socialism or Barbarism basically. This includes CN, pretty much what the KMT did. They were based anti-imperialist early on, whichd allowed CN to grow and fight off the imperialists, but then they started to show their fasc character as conditions solidified and had to be put down to allow for the CPC to flourish.

USSR backed them due to the National question and a issue of survival, it wasn't ideal of course.

KMT was primarily pro national bourgeois and didn't really give a shit about the proletarian or class struggle.