if you wanted to invest in a massive trade navy, is there any advantage of having longships instead of the regular tradeships the westerosi and essosi use?

Unless you’re dealing with a situation where you want to trade up a riverrine network where ships with deeper drafts can’t go (or if you’re trying to set up a smuggling network where you can’t dock in a normal port and need to beach and then ship out in a hurry) no there is no advantage whatsoever. 

Longships aren’t that suited to trade – they’re very small vessels with very limited holds so you can’t carry very much, only one sail as opposed to two or three, they’re very low to the water so rough seas run some pretty serious risks of swamping, etc.

By contrast, your galleys, galleases, cogs, carracks, etc. are much better suited to trade: they’re much bigger so that they can hold more per trip (which leads to economies of scale), they have multiple sails in addition to often having more oars so they’ll generally be faster than longships (which people often get backwards), they’re higher out of the water so they can deal with rough seas more easily, etc.