Warning: I know nothing, I'm from Halmstad, but isn't China completely dependent on selling unimaginable amounts of cheap crap to the rest of the world, in order to feed their population (no matter how many of them there really are)?
Yes.
And isn't it reasonable to think that as long as that market isn't threatened, they can be trusted, sort of? Trusted not to rock the boat too violently, at least?
What counts as rocking the boat too violently? Theft up to and including military technology apparently isn't rocking the boat too violently. Nor is running secret police operations on foreign territory, or using potentially lethal force against ships in international waters, or lots of other bad ◊◊◊◊ they're already doing.
The other issue is that dictatorships sometimes act irrationally. China
shouldn't get too violent (for some definition of "too violent"), but that doesn't mean they won't. They
shouldn't try to invade Taiwan, for example, but they might. Dictatorships suffer from an intrinsic information processing problem. In order to control the population, they have to control and corrupt the flow of information to that population. But that ends up corrupting the flow of information to the top as well, even if they don't want that. That's why, for example, Putin thought he could conquer Ukraine in a few weeks. So China could easily convince itself that the consequences of something like a Taiwan invasion (or opening fire on a Philippines naval vessel, or....) would be very different than the actual consequences.
And that's not even the end of it, because there's also an agency problem. Domestic concerns (fear of a coup, the need to rally the populace around an external enemy, etc) can drive a dictator to take actions which might benefit them personally in some way even at massive cost to the country as a whole. And given the opacity of dictatorships, from the outside it's generally impossible to predict how such internal concerns might manifest in external actions.
tl;dr: China will
probably behave relatively rationally, but that rationality includes a lot of bad behavior, AND there's still a significant risk of irrational behavior because they're still a dictatorship.
I suppose I must add that I have no love for China, for their authoritarianism, or their state terrorism against minorities, and so on, and on, and on, but I would trust them over Russia, for instance.
That's a very low bar.