The Mercenary faction is easily the most disadvantaged in the game when it comes to treasure running since the Mercenary keyword prevents ships from docking at their home island. However, a sneaky way to get around this is Bianco's Haulers, who lets you unload gold when you are within S of your home island.
The Merc sub Slipstream can use crew of any nationality, and is thus equipped with Bianco's Haulers and Master Bianco. Since they are linked and Master Bianco gives +1 cargo spaces, the net bonus gives Slipstream four cargo holds. The idea is to move her out with Hidden Cove and have her load up at a wild island. Then she can submerge and head back home, almost unstoppable.
Santa Molina is a tidy little four-master that I had never noticed before. She offers quite a lot for her cost and becomes quite a threat when crewed with canceler Christian Fiore.
This fleet is vulnerable against a runner swarm and offensive events, but would probably hold its own if facing a combo fleet. In any case, this is the Merc faction we're talking about, so any chance of victory at all is a great accomplishment.
I wish I had come up with this before the end of the fleet contest ...