C919 should start test flights with China Eastern starting Dec 26. It will need 100 hours of test flights before revenue service. As I said here, C919 is now the only alternative to Chinese airlines in the narrow body space.
If they don't order any more MAX (which looks likely), then Chinese airlines have to aggressively work with COMAC to improve the availability and improve the serviceability of C919. Otherwise, they will have no choice but to accept high pricing from Airbus or high lease prices for leasing firms.
As much as we obsess over IC industry on this forum, the reality is that building a modern airline supply chain might be even more complex. There appears to be nation wide effort to create a fully domestic supply chain right now. That will also need plenty of domestic support to be successful. At a minimum, I think all the domestic ministries need and leasing firm need to be forced to make orders for fully (or mostly) domestic C919 in order to have operators to verify their performance.