the scenario in which China just stops selling to the US would probably collapse China's economy too

While the US may be one of their biggest single customer (representing about 16% of China's exports), they're far from China's only customer.

https://en.wikipedia.org/wiki/List_of_the_largest_trading_partners_of_China

The EU is almost the same size of market as the US, and the southeast Asian countries as a group are a larger customer than either of those.

Cutting off the US won't cripple China. Plus the Chinese government is very willing to play the long game if they can foresee an eventual positive outcome for themselves.