Maybe make it that BD need to investing in creating a proxy entity first like by giving up some system and create a unique pirate that's friendly to you but hostile to all then you could use them to raid without war against other empire.Plundering another government's people and territory is inherently an act of war, so I don't see how the idea of plundering without it being an act of war could possibly be coherent even within the game-format that Stellaris provides.
Basically creating proxy and invest in deniability to unlock raiding stance.
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