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More resilient would be nice if it means pirates cannot rollover and consume worlds. However, an extra fleet would not be enough - it is still less than three endgame expedition fleet gang against a player colony. A player that can sat bomb worlds with their Ordos smasher fleet will probably be powerful enough to roll over that extra fleet without much trouble.
I don't want to go too much OT here (perhaps I'll refine my ideas and open a topic in suggestions) but while a detachment fleet is easy to blow up, and a starfortress on its own is also not too challenging, a fleet with extra and better officers that
always stays in the starfortress range would mean that you'd have to fight the fortress and the fleet in the same battle. So I think that would be quite a different nut to crack, for a similar reason why you (normally) don't attack a nexus if an Ordo is still in its range... so the extra defenses together with the proposed +3 stability and +20% bonus for capital worlds would mean that places like Culann or Chicomoztoc would be very unlikely to deciv due to pirates
Right now you can just jump into a star system with transponder off, which causes the defense fleets to come suicide themselves at you, leaving the fortress exposed and alone. My solution would fix that, because this extra fleet would basically never give you chase, just circle around the fortress.
I destroy bases when the bounties pop up for income frequently , but I definitely don't kill them all (I generally explore and then kill any bounties that are nearby), and I've definitely never gone to a system to fight raiding forces. I see raids happen sometimes, but usually things just give some phat request missions and eventually recover. I still don't remember seeing more than 1-2 planets decivilize in any campaign. Maybe I killed enough bases to prevent that on my vanilla runs? Seems strange.
Depends on what system is being targeted... e.g. Askonia can fend for itself even against larger pirate fleets, so basically you can pretty much ignore it, or farm the bounty when posted, cause it isnt likely to deciv, while Galatia star system will get rekt pretty quickly.