Yes I know that some of them were good at piracy, but still I don’t think it should be a major theme of this civ, they were still more oriented to the land than to the sea.
A piracy-themed civ would be more suitable for the Chams, who were quite active around the South China Sea at this time.
I already made a civ design for the Chams, you could check it if you like: Civilization Idea: the Chams (2023 Update) - Age of Empires II: DE / II - Discussion - Age of Empires Forum