P.S.S -- I'd be thrilled if you could start collecting bounty from other players once you've overtaken a pirate base. Kind of like you set up your own pseudo pirate facade
I'm pretty sure you do. I've seen bounty counts rapidly depleting as my ally beats on AIs, for example.
The pirate fleets as I recall 'port to the nearest star (or the pirate base, then go to the star), and then work their way in from there. They'll target the nearest, least-defended world and priortize trade targets over defenses (ie, trade ports, trade ships, etc.) so you can easily defend against them that way.
I think a better alternative would be as others have said - have the raid build in strength gradually (kind of like a Returning Armada; a bit of the raid spawns at the pirate base every minute or so) then dispatch once it's at full strength. Then make the timer tied to the raid strength and vice versa, so if you launch a counter-assault/early offensive and wipe out some of the raiding group before they dispatch, the timer increases again.
The pirate defense fleets do get bigger, I believe it's based on time or player fleet sizes. If you leave it too long, it can end up
like this.