I suggest a priority system for invasion target with factions prioritise in retaking lost original planet in their home system first even if they are not hostile at the moment then aiming for other planets (including independent/depends on trait) in their claimed system or system that they has at least 1 planet so that they will have a strengthened base of operation. This will prevent a cripling moment, in case, the first planet to fall is the only one that has high command and they suddenly turn friendly with each other. The rest will have zero protection agaisnt raid and everything without permanent roaming fleets. There will be a single home system for every faction that also has highest aggressiveness in retaking, all other claimed system are interchangable between faction. They should also start claiming closest hostile faction systems one at a time for they will have a very clear goal to strike with said system turning into a full battlefield with one side trying to claim multiple planets and one side trying to retake and not pick a random planet litter all over the sector to capture (sounds like Stellaris). If the war weariness reach a certain thresshold, one faction will likely to lose the claimed system with reduce retaking and invasion capability.
Some of a very detached faction trait should also has very agressive retaking fleet to ensure their longivity. With this, in hope that, the full picture of the sector will more likely to be preserved, no faction wiped out too soon before player act and faction planets will be more likely to concentrate as to prevent lone planet easy capture.
They should also prioritise colonise a planet in their claimed system if it has 150% harzard and lower. A decivilizing planet should sometimes turns independent instead.
Is it possible to make factions prioritize colonizing planets that are close to them instead of, as the Hegemony did in my game, travelling dozens of lightyears to colonize places when there's better planets available inside one of their controlled core systems? It's really frustrating to see the NPC factions colonize places that I might be interested in instead of the places that would be better for them and that I wouldn't colonize anyway.
Yeah, I think I'll make factions prioritize conquering whole systems before trying to get a presence in new ones (though I probably won't make this a strict rule, might make invasion targets too predictable) and make factions strongly prefer in-system colonies.
I'll also extend the grace period during which no invasion points can accumulate. Right now it's 15 days, which I expect it's functionally irrelevant.
I won't give factions free fleets to retake lost markets (if that's the proposal) though, there's enough issues in the game with fleets out of thin air as it is.
(as for decivilization, I can't readily modify that part of the code at present, but if I ever do I might do some stuff with it)
Could the Tri-Tachyon be given some kind of buff in the standard core worlds start? In my last game the Persean League took their only heavy industry world very early, which completely crippled them, and it was only through me capturing and gifting multiple low defence pirate worlds (including Kaptain Starworks and it's heavy industry) that they managed to survive until the end of that game.
Hmm they already get some fairly significant buffs (both the Culann and Eochu Bres stations get alpha cores) and I'm wary of piling on further buffs on top of that to turn Hybrasil into some ultra-fortified system. Well, I'm giving the military base on Culann an alpha core too, that'll help.
Is it intended that NPC faction-backed rebellions fail every single time? Across an entire playthrough I've never seen a single rebellion succeed unless I actively aided it, which seems like a waste of the mechanic? Not sure how you'd go about changing it, though.
Well rebellions aren't generally expected to succeed without outside help.
If the faction aiding the rebellion could smuggle in such help (like the government faction can call in suppression fleets), that would help them and also add another point of possible player interaction. I'll see about implementing that.