"secondary China routes from SFO are lacking compared to LAX, JFK, and YVR"
Prior to the pandemic, SFO was connected to Qingdao, Wuhan, and Chengdu with Guangzhou, Beijing, and Shanghai all having multiple frequencies. And United even tried to make Xi'an and Hangzhou work for a period of time. So I'd say SFO is fine on the secondary city front, even with the strong demand to HKG. I'll be excited to see what comes out of SFO in a couple years as United gets creative with expansion.
Regarding Hainan flying to SJC vs SFO - Hainan started flying to SJC before China relaxed their "one airline, one route" policy (~2015?). They couldn't fly to SFO at the time even if they wanted to because Air China has operated the route for the last several decades, so they only had the option to fly to SJC.