If you want a reference point at the upper structure limit, consider Wrath of the Autarch.
I've only ever managed to keep a group together through year 2 out of 6 with it, but it doesn't significantly expand in character at the upper end.
Now, Wrath of the Autarch goes pretty dang hard in the paint on stronghold management. Whenever the stronghold isn't onscreen, characters are always asking "but what about the stronghold"? It's the story of how a small breakaway region mustered its strength, forged alliances, claimed power from the wastes, and broke its original empire's hold before the empire could muster its strength and crush it. I don't know how casual your players want to be with their little planetoid base, but if they're up for some amount of exploring the wastes, eluding ancient guardian mechs, talking to a space worm about friendship, and riding it on a collision course with Pirate Lord Hatarr's flagship, it'll probably be a more definite place to start.
The basic play structure is 6 years of 4 seasons each. Each season is an admin phase, a mission phase, and a forecast phase.
During the admin phase, you get resources from your land, and spend them to feed your people, muster your troops, improve your stability, and progress up the tech tree.
During the mission phase, you do a thing. This can be exploring the wildland to expand your settlement, improving relations with a trading partner, waging war to knock down a stock of troops, or quashing something in the forecast.
During the forecast phase, the GM decides if the empire is going to do one of its two schemes for the year. It's got a variety to pick from and unlocks more powerful ones as the game goes on. Then there's a crisis check, where you try to get better than -3 on 4dF for each of your settled regions and trading partners, and if you can't, there's a crisis. Much like modern XCOM, anything you don't address in the next mission phase will mess you up.
That's pretty much a short session, so it's largely bookends on the normal course of play, and it doesn't feel very much like playing a single exchange in a conflict to bonk on a stress track. It's more of a gradually building foe with a timer: year 6, the empire finally gets the ability to mobilize its troops and crush you, and either you meet them on the field to stop that or you take them down beforehand by fomenting revolution, questing for the secret heart of their power, or sneaking into the capital with an armload of poison daggers and finding the right back.
You'll need to adapt: The setting, definitely. You can pretty much get away with subbing in sufficiently advanced technology for magic, assuming that flies with your group. You're also need to come up with a threat and what it means to win for good. This can be as simple as Pirate Lord Hatarr and the Raiders of the Crimson Eye, or your own space empire of choice, or the threats can just be horrid cascading hazards from outer space and you need to get your colony into an alliance or make a technological breakthrough or something.
The choices in realm management are pretty simple. What do you build, and what do you do? Do you put your people on half rations to replace the unit of knights that fell in battle? Do you outfit yourself with better weaponry, or do you put the resources into developing a militia that can let you ignore crises? Do you take territory to get more resources, improve relations with a trading partner to get better deals for your resources, or weaken a target with sabotage or assassination to take advantage of it next time? Can you afford to ignore crises and the Empire's plans to make it happen?
Also, mission structure includes a more free-form prelude that leads up to a more structured confrontation, but the prelude's on a timer. Not dealing with it when the timer runs out gives the GM more fate points to use in the confrontation, but the prelude's also where you can most readily try to affect campaign-level aspects. It's a tough roll and it takes actions you might not have to spend, and of course when everything's been going well there's that siren call of "hey, maybe you can spend some time swinging for the fences".
You'll need to adapt: You'll probably have to change something about warfare. The easiest jump for me is to have spaceships fighting a space war in space, and maybe also certain parts of low orbit. I also don't know how your group's going to take structured preludes. Maybe well, maybe not, but if you drop them for a more conventional windup you'll also need to find some way to limit how much people can roll at campaign aspects. Maybe just don't let them at all?
Food, timber, ore, mana, luxe. Every territory produces some; expansion territories have a total number equal to their difficulty minus 2. Get 'em as d6s, trade some for others, roll and allocate dice to projects or to not starving. Burn a luxe die to reroll as many dice as you want in one other resource. Like your 4 ore dice that somehow all came up 1s. Now some are 2s instead! Progress.
Every tech tree has its own mix of goods, but usually tends to favor one and not need another. Researching magic needs a lot of mana but never ore. An army marches on its stomach but doesn't need mana. (The magic tree has its own units.)
You'll need to adapt: Yeah, this is gonna get heavily reflavored. People still need to eat in the future, but future technologies have their own demands. Food/science/industry/dust/influence? (Of course, if you recognize those from Endless Space you probably have more flavorful names.)
The Realm's Traits
Well, you're probably not going to win a war with starting militia. Certain developments just snuff out crises before they can happen. You can get more resources by taking more land or building up trade relationships, but either way, having resources is better than not having them.
I should mention what "getting yourself better weapons" meant up in the choice section. It's a familiar refrain across the tech tree. It means unlimited rerolls.
Well, limited to once per roll. Standard rules apply there.
And the next level up from that is Fatevantage. Best of two rolls. Only works out to +1 or so on average, but super unlikely to come in subpar.
Different in effect and character from a standard character stunt, right?
You'll need to adapt: The tech tree. Autarch skills are cut down even from standard Fate so you'll need to figure out how your game's skills work into the tech tree.
Here's where you might have a few quibbles. Autarch leaves everything outside the settlement in the GM's hands entirely. The various free factions don't get their own equivalent to this action phase. They don't try to tech up or take their own territory or sabotage each other or the empire. It's certainly possible to model the GM's twice-a-year scheme interventions as one faction making a move on another that's going to have fallout for the players, but that's different from the players being one element among a bunch of GM-run dummies bouncing off each other.
Each faction has their own character, of course. Some places are easier to sneak into than others, some are more amenable to diplomacy, everyone has their own war complement (and allies participate in each others' wars).
You'll need to adapt?: This choice is something that works with the other limitations - the players get four missions per year, two of which are in theory unencumbered by schemes. This is a deliberate emphasis on player-facing choices and impacts of player actions, which is a bit harder to pull off when you're outnumbered six to one.