Everyone gets bored of the invasion screen, sure. I myself have thought up dozens of ways in which it can be expanded past the "two lines going at each other" paradigm. The problem, however, is that if you make it any more intricate it might make invasion too much of a chore. If I'm going for a conquest victory on a Huge map I don't necessarily want to spend half of the entire game time managing ground battles on different fifty worlds. It's really all-or-nothing; if the game doesn't implement a full Command & Conquer-style strategy game for ground battles then they must necessarily allow an element of randomness to simulate everything that happens, and once that element is there they might as well boil it down to its most basic context, which is army vs army with the dice slightly weighted toward either side -- and this is exactly what the invasion method is right now.
That said, it's entirely possible to imagine an invasion mechanism that is simple, interesting, interactive and not excessively time-consuming. Realistically it's difficult to reduce war to fewer factors than technology, troop count and invasion strategy, all of which is taken into account in the game already, to a degree. One might be given more choice, though: pre-invasion orbital bombardment of farms could decrease the defender's troop count; likewise destroying their labs and factories could damage technology. More in-depth strategy choices could include things like surprise attacks, surgical strikes, turtling with artillery, even guerilla warfare -- all useful if you're going in with fewer troops than the enemy has. On the other hand, if you have poorer tech you might have to resort to hording, scorched earth, terror warfare, espionage, and simple brutality.
Lastly, one might be given the option of deployment: imagine being given a map of the world on which each square has a troop count and tech rating, and you as the commander can deploy all of your invaders in a single square or split them up. Again, conquering an industrial sector will decrease the defender's overall planetary technology and troop count in a 1:1 ratio; conquering labs will drop it in a 4:1 ratio; nailing the capital might drop it in a 1:4 ratio. This might mean that wars will be fought one square per week: class 4's will be done in a month at least, and bigger planets will take quite a while longer. Mid-war re-inforcements will then me entirely possible.
Just a few ideas.
J.
