I was told that Chinese airspace was quite restricted. Only a few authorised tracks for international flights.
FWIW that's pretty much what we're told by our planners.
In quite a few parts of the world access to routes are regulated by various agreements and treaties, but Chinese airspace in particular is very rigidly controlled, to the extent that it can sometimes be difficult, (and in the past was almost impossible) to even negotiate a few tens of miles/kilometres lateral deviation from an airway centreline to get around thunderstorms. There's also only a limited number of routes across Mongolia and also a limited number of places where Russian ATC will accept entry into their airspace.
FWIW there is sometimes the option of a route that passes in the vicinity of Lanzhou and then eventually heads west to Urumqi - that's certainly a route that I've seen used going eastbound.
If you try routing much further south and west you will eventually run into problems with overflying significantly high terrain for an extended period of time (the "what if's" come into play: e.g. engine out performance, sufficient availability pass/crew oxygen in the event of a decompression).