The empire's purpose isn't actually to conquer. Emet said in ShB its true purpose instigated by him is to cause chaos, and they can't do that if all of their opponents are dead. Even if everyone except the imperial family doesn't know that, the way the empire is set up and organized fuels it. Like there was no logical reason at all for Zenos to have been viceroy of two regions clear across a continent from each other (Ala Mhigo and Doma), that was just done to cause chaos.

That aside, Garlemald is absolutely not in the space age. And things like the Weapons are experimental prototypes, not something they can mass-produce. The good guys also aren't only fighting with sword and board, they have magic, airships, and other form of machinery even if not as much as Garlemald does.