Is it possible, if you want to delay an invasion force for just 1 or 2 turn (in order for reinforcements to arrive) to simply remove your Orbital Strategic Manager? That way, the attacking AI would have to use at least 5-6 movements to take care of your whole fleet, when it would simply take 1 usually.
That way, if you put all your transport in the same square, but don't fleet them, you would never loose more than 3-4 per round, while you would lose them all if you had made a fleet out of them
