They can apply enormous tariffs, or even cut off the flow of trade into and out of our civilization. They can allow our (future) enemies to use the strait to invade our shores, and deny us the ability to counter-invade them. They can act as a financier, a supplier, intel source, pirate haven, rebellion inciter, or coalition builder.
Simply for trade leverage? Who's to say that they will always be content with only controlling the strait itself. In the future, they may have designs on Anatolia, or our Northern Coast. Not to mention that the control of trade is absolutely vital to their existence. If they think that we are nearly powerful enough that we can force their tariffs lower, or even bypass them entirely, they will have no choice but to act against us. Just like the Highland Kingdom and Thunder Speakers are currently our non-warring enemies because they feel that we have too much power in the Lowlands.
And how will the Trelli fight against a superior force? By allying with our other enemies, or by using their immense wealth to hire mercenaries. They may not look threatening now, but I wouldn't want to face a coalition of the Trelli, the Highland Kingdom, the Thunder Speakers, the Thunder Horse, the Swamp People, and any of the Steppe Nomads and Balkan/Hungarian tribes that they are able to hire.