Citing one of my favorite philosophers, Aristotle, and the one that is my favorite book of ethics, Nicomacus Ethics, virtue is the just (or point) half between two vices, one by excess and one by default.

What defines the standards of what is considered excess? Go back a thousand years, and nearly any modern government would be considered "excessively" democratic. Or, look at the debates over slavery in early America. The idea of immediate abolition of slavery was the most extreme position then, yet it's considered a given today. Looking back at all the compromises made to "keep the peace," it seems like they only served to prolong the institution of slavery, and that conflict was necessary and unavoidable.

If we really want to play with the idea, I wonder if the Golden Mean implies that nonbinary people are better than everyone, because ideally you want to be balanced between masculinity and femininity. It's pretty clear that we cannot apply the Golden Mean uncritically to all possible qualities.

If the objective is to guarantee the best possible conditions for its population, which I do not understand why they do exactly the opposite

They don't. You're painting a one-sided picture that only looks at the negative. They have eradicated extreme poverty, they invest in massive high speed rail projects and develop green energy. The average Chinese person has seen massive improvements in their material conditions in their lifetime.

Much of what you're discussing are political freedoms. It's true that I have more freedom to talk shit about my government than people in China do about theirs. But all I can really do is talk shit. The politicians aren't going to care how much I criticize them, they're bought out by corporate interests, and many of them are in safely gerrymandered districts. At the same time, economic conditions can restrain one's freedom as well. My right to shit-talk my boss doesn't mean much if my boss can fire me on a whim and I need the job to survive. Is being able to whine impotently about politics really worth having to follow whatever rules a landlord sets for you because you can't afford your own home? Meanwhile, home ownership in China is over 90%.

With all that being said, China's system is not my ideal. However, I do believe that China's existence is necessary for more ideal systems to emerge around the world. Without China, the US's status as global hegemon would be completely unchallenged, and it would be all the more free to ruthlessly suppress any remotely left-leaning political projects, anyone on the globe, through sanctions and coups. But, the more ruthless and aggressive they are, the more countries turn to China, because it exists as an alternative. Even if China were as bad as the US, it would still be preferable to have power divided between two superpowers than one.