[Fyron]

You could make ground combat last only 1 or 2 rounds. This way, there is a good chance that multiple transports can be used without entire loads being slaughtered and the militia automagically being regenerated from nothing within the same game day...

Make militia have a lot more hit points too. These changes will allow the defender to drop reinforcement troops as well, as it will likely take several game turns to finish conquering the planet. No longer does ground combat have to resolve in a single round...