While I don’t think it bears much on how reasonable it is to suggest nuclear powers agree to never strike first, China’s arsenal is uniquely well designed for this kind of strategy. They employ zero static sites, unlike the US and Russia, relying on mobile launchers, subs and bombers. This makes them tactically poised for a retaliatory strike as they don’t have as much of the risk of losing their launch sites in a first strike. The US doctrine of preliminary strike in the event a nuclear attack seems likely is designed to protect their ability to launch at all.
While this kind of treaty would be slightly “advantageous” to China, it’s only because they set up their nuclear arsenal with this far more reasonable and less aggressive strategy in mind from the get go while Russia and the US would have to adapt and convert their arsenal.