The Cardassians were duplicitous and dangerous empire that would go on to become a part of a more dangerous and destructive regime. And there's no telling what devilish schemes the obsidian order would have concocted even if the wormhole wasn't open. Full conquest of Cardassia and installation of a democratic government, as well being put on track for Fed membership would have been in everybody's interests including the Cardassians.
None of which comes under the criteria of a just war. In fact as noted Jus post bellum specifically precludes annexation.
Jus ad bellum (pre criteria for waging the war) requires that doing so fulfills a just cause (specifically to prevent the imminent loss of life) and also meets the criteria of last resort.
If you are arguing that you believe such a conquest to be a moral action fine (although I'd disagree), but the term "just war" is an internationally recognised one with a significant pedigree and a core set of principles and criteria. It isn't a throwaway term intended to be used ad hoc according to one's personal taste.