Then wouldn't they be merchants, not pirates? And if one wanted to improve that, some sort of spell to control and manipulate others would be a better option due to a wider scope.
Thieves Guild
Rob from the rich and good, give to the clients and cult. The Thieves Guild is a collection of the worst sort of thieves, lowlifes, smugglers, scammers, and murderers, led by Warcasters and Accolytes who likewise fit in. They'll accept any job so long as it pays well, be it murder, arson, theft, sabotage, or framing. Their main advantage is the DDD spell that a caster in a team can cast for their allies, allowing to get into any building, structure, or stronghold. As well, the development of MD gives them access to potent poisons for use in operations, and mutagens for panic when they're hired to cause that or need a distraction. Likewise the ravens create an information network that allows them to seep into society with their knowledge of the guards' patrol paths. So long as there is coin, you can hire the Thieves Guild for any underhanded task.
The hideouts themselves have the Language of Inception painted around them focused on bringing those that see them into deeper sin, and the symbol for the Guild itself has runes meant to lead others to find them inspiring, helping with recruitment. In places where a Thieves Guild already exists, we just need our consorts and tactics to draw them into our cult's control. Likewise, we will subvert other local criminal groups as needed as well to help with our goals, and deal with those who our allies say need dealing with.
Found it.