And none of these even approached the cardboard idiocy of 'for sport' or 'just to annoy someone'.
Oh, don't take me too literally. "To annoy someone" is a valid way of describing why the US wanted a strong postwar Germany in Europe, even if a bit flippant. It's exactly the sort of game the Romulans would play. And "for sport" is what conquerors do for valid strategic reasons: they need to keep giving their forces manageable "baptisms of fire" against foes of optimal strength before tackling their "real" opponents, and Klingons and Germans have realized this equally well.
Cardassian territory/etc IS the 'true goals'. Spoils of war.
This does not appear realistic at all. Cardassia is the only expansionist power in Trek described as having a profit motive, a need for land. The other powers do just fine without having to worry about new worlds and civilizations. France had a massively expansionist empire a hundred years ago, yet no desire to acquire any land in Europe; even Nazi Germany lacked plans for exploiting the conquered areas, beyond the single vital commodity of oil, and was interested in conquest only for the purpose of denying its enemies the strength inherent in the land. As already mentioned, this is a "fashion" thing, something empires sometimes do and at other times do not. We have no objective or a priori reason to assume either of these options for the TNG empires, but we can witness them in action and then decide.
Opportunist exploitation of resources is common in Trek: many a plotline revolves around a "secret slave mine", for example. Yet e.g. the Klingon actions in the conquered areas before the war wasn't described as resources extraction: rather, it was indicated that the Klingons pumped resources in
, in order to fortify their holdings.