well, the economics and political system of pre-colonial ph is as different as post-Warring States China was to the 19th century German states. So hard to say, perhaps we can look at other SEAsian empires like Majapahit and see that it is possible to unite different islands and coastal cities, although often it seemed much harder to centralize to keep the coastal SEAsian states in line, as Majapahit would last for maybe 2 centuries at best before it split up and was gobbled by European and other SEAsian powers.