[Mayday]

Thats the IG2 method. Excepting of course that that was fully 3d and required all planet to space weapons be destroyed before landing.
Of course, this would limit the planetary takeover time to just that battle. Though in such a technological era, we can expect that the victorious force would have devastated the resisting military into surrender.
In order to make this a bit more balanced, I suggest that the population resist, ala Civ3, for a length of time decided by the population size (causing riot status), and that this would be offset by having the military presence there. This should obviously be a changeable setting.
Also, the demeanour of population in pop transports should be recorded so that one simply couldn't lift most of the population off a planet to shorten the time to sort out complete order. Indeed, putting the population back down should send them back to riot status, or at the very least Angry, with a military presence there, if the average happiness was lowered enough.