Stealth is not invisible. This is a very important point, tiny RCS (Radar Cross Section) is just that. Tiny, but still there. You just have to get closer to find/track the stealthy object. For an object lesson ask the F-117 crew shot down in the Balkans. Still, clever tech (but it will bite you later)
People would notice the big convoys traipsing off into the asteriod belt, sure the solar system is big but not
that big. Sure you mine asteroids but you need a crew. And lots of materials you aren't going to get from asteroids. So you need freighters, which all the clever Icaran sensors are going to pick up. (Told you it would bite you
)
If your keeping ESS and Icara still not happy (or grudging but with occasional clashes) that works alot more for me.
A flintlock is complex, as in massively so. The first mass production was called 'Armoury practice' because you needed standardised parts to build and repair at any speed. The tolerances and metallurgy needed was the results of decades, even centuries, of poking and improvement of match/wheel lock rifles. It's not just gunpowder, it's not even the metal working technology, it's the skills and equipment.
Resistance to foreign occupiers is almost human nature, unless the current regime is hated, even then the occupiers aren't liked. Rome was the lightest of imperial goverments. Pay us a levy and don't attack our troops. That was about it. The fringe benefits (roads, infrastructure in general, etc) were also great. They still weren't liked. And like all empires it declined and fell to paraphrase Gibbon. So an empire lasting millenia, that didn't face major rebellions does seem odd.
Finally, because I like to end on a joke, The underground Nazi base. So they'd be the the hollow earth Nazis not the space Nazis based on the dark side of the moon? It is very sci-fi cliche you must admit.