As you want, i won't bring you into a multi-hours debate, but i.m.o. :

China has plenty of diversity across provinces

Yes, more than in the u.s., but less than in a confederation of city-states with their own constitution(, and a small common set of base laws throughout the confederation).

Doing a referendum across 10 millions people makes a lot less sense because those people likely don’t have a lot of overlapping concerns in most cases.

Local/Municipal referendums for local decisions(, or even more wide-ranging as Venezuela and other socialist countries showed with their communal councils that vote on the budget and the rest).National referendums for decisions that affect everyone.I may have misunderstood your point here, so forgive me in advance.

Do you need more buses, or more roads, or LRT, or subways, or all of the above.

I'm convinced that hearing the different sides of this debate(, in a municipal assembly here,) would be enough for citizens to make the decision.You're convinced that citizens wouldn't be able to make the correct decision.How can we know who's right here ?

At least there wouldn't be corruption by building companies that divert millions each year for relatively useless construction projects whose main purpose is enriching themselves(, private-public partnerships likely are more often a scam in capitalist-owned countries than in the p.r.c.).I've got in mind one of the rare examples of direct democracy in a very small village in France(, a few hundred inhabitants, called Ménil-la-Horgne), where they ended up building a small dam to store their energy. That's a real-world example in which they successfully came around together on a solution to diminish their carbon emissions, after weighing the different alternatives.And, again, it doesn't take that much time, for Ménil-la-Horgne i.i.r.c. it was something like four or six 3-hour reunions per year(, i don't remember exactly but it was in that range, perhaps less).And that's scalable for larger cities, e.g. by organizing it into districts(, and after all Athens was quite large already). Another option would be to leave the vote to citizens drawn by lot, forming a mini-society representative of the other citizens, as long as they're not manipulated of course, and eventually afterwards a confirmation of their decision through referendum.

(Also, it's my belief that in our current system, our "leaders" and deputies shouldn't be paid at all(, or only the minimum to survive whenever their bank account force us to make an exemption). It's unbelievable that some are here for the money or power, wtf frankly, they should be here for ideals and nothing else)