Some ideas and comments:
- To archive plausible deniability, factions only send mercenaries/hired pirates on expeditions, as long as your are not hostile (or vengeful) with them. Nothing you do to these fleets has any impact on relationships. If you are hostile, they send their own (stronger) fleets.
- Pirates don't usually attack a well defended colony, they focus on raiding transport fleets. If pirates attack a planet, they were usually paid by someone to do it.
- You might find hints on their employer in the wreckage of a defeated pirate/mercenary fleet, or at a bar at their place of origin. You can use that evidence to pressure the faction and gain reputation/monetary compensation.
- The frequency of expeditions scales with your relationship to a faction.
- In general, the frequency of expeditions is much reduced. A reduced market impact of (fledgling) player colonies should be a main reason for that.
- The player raiding a market with their identity known (transponder on) greatly reduces relationship status, it's an act of war. You have to sneak/battle your way in with transponders off, or hire mercenaries/pirates yourself.