The Homeworld series is great with fantastic campaigns (minus Homeworld 3 I’ve heard, not even played that).

I’ll also throw in a classic Imperium Galactica 2 because I still think for a 90s 4X RTS it has so many elements that I’ve just not seen replicated since. Though usually short and quick, it has fully simulated and controllable space and ground battles; espionage; diplomacy; you assign your unlocked tech to hard points on your ships… It’s Stellaris but better in most ways, imo.