Between 2010 and 2019 we visited China 4 times. Classic China: Beijing-Datong-Pingyao- Xi’an took flight to Guilin : Ping’an,Yangshuo, villages with the rain and wind bridges duren golden week October flight to Shanghai world exhibition and Suzhou.
Silk Road .
Shanghai to Guangzhou
Yunnan and minorities in march.
The other 3 trips were in September. From Beijng till Guilin we had constant rain and cold weather.
We want to re- visit in 2026 Beijing-Datong-Pingyao-Xian/Luoyang-Shanghai-Suzhou and the neighbouring water towns.
We can travel any time being retired.
8 may for 30 days flying into Shanghai and departing from Beijing or 8 October starting Beijing ending in Shanghai 30 days later.
I avoid September for Shanghai because of tyfoon season.
Which period would you suggest to have the least chance of rain and clear sunny skies? Or other timing?
As Belgians we don’t need visa if we stay maximum 30 days.

Have you looked at weatherspark.com? It has excellent climate date, including info about rain and sun. As for other factors, I'd check holidays.