Koxinga dies before attacking the island. Then, without the presence of Ming loyalists on the island, the Qing aren't motivated to invade.
As for not being sinicized, I doubt we can avoid an important Han population. Chinese traders from Fujian were already present and the immigration was encouraged by the Dutch to develop the island's agriculture.
The main difference would be a more pronounced and vocal Aboriginal community thanks to Dutch efforts (they had already begun romanizing local languages for missionary purposes).
And there would also be, I think, a sizable Dutch community.
The Dutch prolonged presence would mean a more important trade with China through trade networks established with Fujian, leading eventually to Amoy being kept open to foreign trade instead of only Canton.
And the proximity of Dutch presence could lead the United Provinces (or at least the Dutch East India Company) to get a diplomatic representation in Beijing, a privilege that only Russia enjoyed until 19th century.
The island would be especially more important to the Dutch as it would provide them what lacked to the British to trade with China, cash supplied by the island exports to China, not to mention of the usefulness of Chinese traders settled on the island.
Then, given the geography and the island own importance to the Dutch, it would be detached from Dutch East Indies to form an autonomous entity for a more efficient administration and defense.
Dutch naval presence would be likely to rise throughout the eighteenth century in order to fight piracy.
The island should be far enough from English bases to be safe unlike Malaya.
Survival of Dutch Formosa is in fact the object of a TL of mine in gestation but that I should only begin after I end the Alaska wank I'm preparing now.