With respect to defeating air defenses, you actually generally launch a massive air attack and try to send aircraft with the countermeasures and maneuverability to have a decent chance of avoiding the missiles. Or if you are china, you try to run them out of ammo I guess (i tend to doubt chinas ability to defeat modern air defenses as they are right now). Sending in ground troops generally isn't especially desirable because their ground defenses are almost always way deadlier than their air defenses, and if you defeat their air defenses you can erase their ground defenses with impunity.
I think wearing down cities at ultra long range should be perfectly viable for star ships. Yes the cities are massive, but they are on predictable trajectories. You could easily imagine some kind of kinetic bombardment weapon that can shell the shit out of cities, and the cities could only return fire against other cities, because spacecraft can just move out of the way of said weapons. Maybe big and slow nuke lobbing systems. Something like that could be a sort of balancing factor against the power of cities I think.
Assuming shields played into this, I think this could lead to some sort of siege mechanic thing some of us have always wanted. That is to say, something that provides time to react to enemy invasion if you can't be online 24/7. For instance, maybe a special kind of shield bubble that can withstand any damage but only for a set amount of time (increasing logarithmically as you build the system bigger so there is some upper limit for any one worlds siege defenses). Then you have to potentially shell a city for quite a while before it gives in and can be taken by force properly.
e: It might be better if the siege shield only really works against the super long range stuff, so that if you really want you can close in to weapon range and try to slug it out against their guns.
I think wearing down cities at ultra long range should be perfectly viable for star ships. Yes the cities are massive, but they are on predictable trajectories. You could easily imagine some kind of kinetic bombardment weapon that can shell the shit out of cities, and the cities could only return fire against other cities, because spacecraft can just move out of the way of said weapons. Maybe big and slow nuke lobbing systems. Something like that could be a sort of balancing factor against the power of cities I think.
Assuming shields played into this, I think this could lead to some sort of siege mechanic thing some of us have always wanted. That is to say, something that provides time to react to enemy invasion if you can't be online 24/7. For instance, maybe a special kind of shield bubble that can withstand any damage but only for a set amount of time (increasing logarithmically as you build the system bigger so there is some upper limit for any one worlds siege defenses). Then you have to potentially shell a city for quite a while before it gives in and can be taken by force properly.
e: It might be better if the siege shield only really works against the super long range stuff, so that if you really want you can close in to weapon range and try to slug it out against their guns.