We actually can't wait them out if they take the West trading post site themselves. That gives them 2 Dominances(Bronze and Tin, which they are already near dominance at), and thus 2 more Wealth worth of Mercenaries.I really think that the Trelli are overextending here, and it's going to bite them hard.
Remember, there are no "I Win" buttons in this quest. If something seems like a universally-winning combination - eg take slaves, get wealth, hire mercs, take slaves - then it's flawed in some way.
Remember, they didn't develop currency and mercenaries themselves. They copied them from us. But they don't have the social foundations of symphony and peace and acceptance that we do. And they haven't formed companies of their own loyal citizens; they've hired outsiders. And they're stirring up wars left, right, and centre, to trade the spoils.
It's not sustainable. Something has to give. For example, the supply of "unprofitable" tribes may be exhausted, leading to a large dip in trade, and suddenly they can't pay all these mercs. Or someone wealthy and ambitious might turn the tables on them and hire the mercs to raid them back, or even capture the city; remember, these guys aren't actually loyal to Trelli.
We can wait them out. Let's build the east trading post, keep them thinking we're their friends, make them rich enough that they'll overextend, but build up our ability to resist pirates, so they don't bother hitting us, and they instead raid the west until it either runs dry or seeks revenge. Meanwhile, we make friends with all the survivors of the climate crisis, so when the Trelli implode, our trade doesn't.
Which is enough dudes that we can't really stop them any more than Venice's neighbors could. They are overextending sure. The next major trade disruption that hits will set them on fire all at once. But until then, if they get the Tin route they're going to eat the western black sea uncontested.
Just consider what happens when they can afford to throw 4 Mercenary bands at any given problem indefinitely. 32 Martial worth of punch.
We did get the same thing from the Hathatyn piracy. Gave us a 1 turn casus belli for "bandits".Also, I'm pretty sure that they don't need to declare war on us to raid our shipping. All they'd need to do is act as a haven for pirates, and the pirates would naturally sell their ill-gotten goods in their city.
We'd probably get a casus belli of course.