Why would you want a harbor on the (relative to China) eastern side of the Pacific?
That's the problem I have. Sure if you're establishing California it makes sense (although even there only if you expect to be trading by sea from that point, as opposed to say OTL Spain's efforts) - but it doesn't make sense to establish California because of the Bay Area.
Not when China has little interest in settler colonies - which are hard to establish and not very profitable in the short run ("generations later" is Civilization thinking, not mortal merchant thinking) even if they survive.
If the Chinese are colonizing the West Coast of North America, their priorities (in no particular order) are:
- Acquiring gold and silver from Mesoamerica.
- Colonizing coastal regions, because those are the most habitable and agriculturally productive in the region.
- Repairing and resupplying ships, who have taken an extraordinarily long journey, even if they already rebased at Hawaii or British Columbia. Also the fastest method of travel is by ship.
- Trading with the natives.
- Defending against natives, as well as possible pirates from Baja, the Sea of Cortez, mountainous coasts in Mesoamerica, or the Pacific Northwest.
Mesoamerican metals are the top priority. Since coastal travel is the safest and Baja only has a few good places to put ports, what's between the Old World and Mesoamerica? California. What's the best, largest, most defensible natural harbor in California? SF Bay.