Isn't most of the Solar system's accessible solid matter located in planetoids? I mean, asteroid mining is all fine and dandy, but even a small planetoid like Mars or Europa has hundreds of times more resources close the surface than minable asteroids.High tech processes usually don't and prefer completely controlled conditions so them taking place in space changes little in terms of that they need to completely control the purity of their produce anyway. So them adding the atmospheric mix they want is just part of the whole idea of producing high quality alloys.
You are assuming a civilization living on a planet. All the problems you describe the civilization would have to tackle when it went into space in style and breadth. There'd be no reason for a civilization to go into space if they haven't mastered zero G building and exploit the raw resources of an entire solar system or considered it worthwhile. There'd be no point in it if escaping a gravity well is still a major problem logistics wise (that doesn't preclude it being a cost factor).
But once the civilization starts exploiting an entire solar system the vast majority of their industry and mining will take place in space anyway because there's far more stuff there than we'll find on Earth and there'd have been a reason to leave the planet in the first place that exploiting it is worthwhile. So I think the presumption that the planets are the interesting bits in conquering neighbors begins to fall apart. As you say, suddenly the places where planets aren't are more interesting because you don't have to haul material up and down a planet's gravity or deal with its atmosphere or deal with an ecosystem so doing it in bulk will be more economical there and you only send consumer goods back to a planet's population.
Maybe they become the rural backwater areas none bothers with because the transport hubs and economy will be where it can easily gather stuff from all around the solar system and beyond; which is not necessarily a planet but maybe simply an orbit around the biggest concentration of raw resources, or a moon with a quarter the gravity and no atmosphere to deal with etc.