This is actually a very interesting topic, and something that is kind of fun to ponder. Unfortunately I haven't had the brain power to make a really good response yet.
I suppose, ultimately, it would work very much like it does in the real world. I suppose space battles would be a lot like sea battles. Distances, fuel, etc. would be a very heavy concern. The Star Destroyers in Star Wars as a type of aircraft carrier seems very well thought out. I'd go a head and say that you likely would need to create footholds, finding a strategic base near enough to the target to be practical for refueling and repairs. If a planet fortifies itself, then an invader would need to break through their defenses before continuing the attack on the ground. The same ships this invader used for breaking through the enemy lines to get to the surface of the planet, would likely not be suited for capturing targets on the ground. So again, SW has some very well thought out ideas with things like the AT-ATs and other war machines designed for surface combat.
To address the pondering of what a "front" might look like, I have always kind of imagined this as some network defense around the planet, perhaps a fleet of ships, or a series of satellites or defense stations that are intended to keep enemy ships from being able to enter the atmosphere. I'd imagine the conquering of a planet from start to finish going as follows,
- Invader sends their forces toward the planet, most likely from a base within the closest distance possible.
- Defending planet consentrates all its resources to fending off the attacking fleet. If it succeeds, all is well, and they probably have a lot of ships to collect salvage from. If they fail...
- Invader takes out all of Planet X's planetary defensive forces, which would likely mean every last ship in their fleet. From here, they head to the surface, where likely a ground army is preparing to do their best at fighting off the invaders. Perhaps there would even be non-space worthy aircraft involved at blasting away at the incoming ships. Also very likely ground to air missles and other such forms of defense. Assuming the invader is strong enough to hold together through such an onslaught...
- Invader begins sending out ground forces to capture important buildings and locations.
I see invading a planet to be a bit like peeling an onion. So I don't think there would be that many cases of battles raging on all layers at once. Of course, it would be unreasonable to think every planet would be so well defended, so likely in many cases the battle might begin on the surface. I'd also imagine that many wars between well developed planets might never even see a ground battle, and that all the fighting would take place in space, as those defenses would be quite a lot to breach.