How confident are you that it's preferable for America to develop AGI before China does?

Interesting question. I think there is a plausible case to be made that convergent factors in AGI/ASI development might render it less important where it came from, and that fixating on this might simply cause dangerous race dynamics. However, it seems pretty clear to me that directionally the US is better:

Over the past 30 years, the Chinese government has done more for the flourishing of Chinese citizens than the American government has done for the flourishing of American citizens.

 Prior to 1979 the CCP was one of the most tyrannical and abusive totalitarian governments the world has ever known. In addition to causing a huge death tool and systematically violating the rights of its citizens, it also impoverished them. Rapid growth since then has largely been the result of a return to more normal governance quality, combined with a very low base. It's a big improvement, but that doesn't mean policy has been amazing - they've just stopped being so abjectly terrible. 

However, at the same time they stopped being so communist, the CCP started implementing the One Child Policy. The US has done some pretty bad social engineering in time, but none with quite the cruelty of the OCP, or whose effects are quite so predictably disastrous. Maybe they will get lucky because robots will arrest their demographic collapse, but on an ex ante basis the policy is simply atrocious. 

Commerce in China ditches some of the older ideas of Marxism because it's the means to an end: the China Dream of wealthy communism.

Responding to this one would take more time than I have so I will skip.

The American Supreme Court has established "corporate personhood" to an extent that is nonexistent in China. As corporations become increasingly managed by AI, this legal precedent will give AI enormous leverage for influencing policy, without regard to human interests.

I'm not an expert on Chinese law, but my understanding is the key parts of corporate personhood - the right to own property, to sign contracts, to be sued, etc. - exist in both China and the US. Perhaps you are thinking of Citizens United v. FEC, but that is primarily about free speech, not corporate personhood, and free speech seems like an area that the US is clearly superior to the PRC.

Compared to America, China has a head start in using AI to build a harmonious society. The American federal, state, and municipal governments already lag so far behind that they're less likely to manage the huge changes that come after AGI.

I'm not sure what you're gesturing at here. 

America's founding and expansion were based on a technologically-superior civilization exterminating the simpler natives. Isn't this exactly what we're trying to prevent AI from doing to humanity?

I don't think that is a fair summary of the foundation of America, and nor do I really see the relevance here. Even if it was relevant, contemporary US treatment of native tribes seems significantly better than PRC treatment of groups like the Uyghurs.

Human rights abuses seem much worse in China - this alone is basically sufficient for me

It's a game of chicken, and I don't really care which side is hitting the accelerator if I'm stuck in one of the cars. China getting uncontrolled ASI first kills me the same way that the US getting it does.