Some thoughts on conquering worlds in Starsector
TerranEmpire seems wise, and I largely concur with what he said. But this is interesting to think about, especially in terms of the population numbers.
Short version: conquest should probably be basically impossible barring little fringe worlds (the ones we actually see change hands regularly in the lore). At least not on anything less than a decade-odd timescale.
Conquest should probably be borderline-impossible for most of the big centres. I mean, they're massive. If significant conquest were easy, it would happen more in the background. Overthrowing the Hegemony should take at least five or ten years of sustained effort after the first shots are fired unless there are co-belligerents with similar resources. Hell, it's arguably unrealistic that a place that saw tens or hundreds of thousands of people move to it in the space of a half-decade (i.e. any player colony) isn't an unstable hive of violence with constantly shifting politics and a ton of people who won't follow the rules at any one time, or that the player doesn't see regular revolts or power-grabs from people who think "well, why NOT me?" Putting together an effective war machine should be pretty difficult.
Even integrating people from a cryosleeper should be hard to do quickly.
Combat strength for a large planet being 200 makes sense because that's its defence against a raid. But a raid is a very different thing than an invasion. I can probably steal stuff from my neighbour's house and run away, even if he's home. If I want to move in there, it will be trickier. Ground strength isn't the total forces. It means that anywhere your forces go on that planet that's worth going to strategically, there'll be 200-odd marines (or equivalent) within easy response time that they need to deal with. For a real-life example, the US has like 1500 odd fighters. Five responded to the 9/11 attacks.
And re rebellions and such, look at the wars in Iraq and Afghanistan - both medium size 7 countries in game terms. Both took the better part of a decade after invasion for their economies to start really recovering. The War in Afghanistan is still described as ongoing, and I don't know that the Iraq War isn't either. In a game where the Luddic Path have crashed space stations into planets and Tri-Tachyon still screws around with AI despite it blowing up in their face AND wars being fought to make them not do it again, I doubt the civilians of any planet are likely to be particularly willing to let themselves be conquered. The first quote for the first mission of the game? The thing it tells you to play right after the tutorial? Smuggling weapons to an insurgency on Volturn.
That said, wrecking the economy is probably pretty easy. Just blow stuff up until it stops working. It's one of the themes of the game. It probably should be easier to control the planet to the point you can destroy the economy (well, some aspects of it) than to the point you can actually economically exploit the place.
Practically, I'd say large planets should be effectively capturable by events only, probably ones specific to that planet, and not all of them at that. Like you could rig the episcopal election on Chalcedon to turn it over to the Luddic Church (or rig others to turn Church worlds over to the Path). Or convince Volturn to revolt with ARCist support and re-establish the Askonian polity. But the player doesn't and probably shouldn't have an army that can challenge any of the major interstellar players other than Tri-Tachyon, certainly not alone - the population depth isn't there, and there's only so much technology can do to balance that out. Tri-Tachyon is a faction and none of the other independent worlds are because Tri-Tachyon has a size 6 world and heavy industry to work with (and a massive technological advantage, plus a bunch of pre-existing infrastructure). Without that, they'd be another set of indeps.
Gunboat diplomacy definitely should be an option for the tiny worlds and stations (if Tri-Tachyon or random pirates can show up with fleets, so can I). Size 3 and 4 are definitely vulnerable to the player. But even size 5 I'd expect you to need local allies and a fairly long timespan to absorb them.