Why would the galleons be a hindrance?
Galleon trade = silver trade. not unless you don't know how valuable the silver trade is for the Chinese back then.
I think you can't elide over the differences by saying "regardless of racial caste." What each group would do in response to a Ming invasion is very different; are you including, for instance, the Chinese population in this "loyal spanish/catholic subjects?"
As of worldmapper site, philippine population in 1500 was 8m. Of course this would change by 1600 due to the Spanish disruption of food sources and trade. the 600k loyal spanish subjects is conservative count since they only count church attendance and tributes.
In otl 1603, There was a Chinese rebellion led by a Chinese catholic convert helped by Mandarin Chinese officials which landed in a fleet of large ships. Although spanish peninsulares were outnumbered by the Chinese, the Chinese were still outnumbered if you count the locals as spanish. You also had the Japanese helping the spainiards putting down the rebellion.
So in a way you can say that there was an attempt by the Chinese to invade the philippines in otl which miserably failed.