So. If I have understood the rules correctly, when you attack somewhere, the order of business is:

One round of space combat
One round of ground combat
Possible retreat
Repeat

(Yes?)

This... in my opinion, doesn't make a lot of sense.

Mainly because this means, that I would invade a planet with two assault transports filled with storm troopers, and win the ground combat, even if the rebels were guarding the planet with several fighters, corvettes and mon-cal cruisers.

I have been using another combat system, wherein the attackers has to win space combat in order to land troops on the planet.

What do you think about this?

Yes that's the order.

Yeah... I tend to agree that the combat order doesn't make too much sense thematically. I think its that way for balance purposes. Usually the Rebel player is pretty out-matched in space combat the majority of the time, but when it comes to ground combat they are a little more even. Airspeeders are annoying . I'm thinking the combat order is to help the Rebels. They would have a real hard time attacking ground forces if they had to win every space battle first.

Combat is the one thing in this game where its kind of lacking.

I understand your point but theme wise this is not true, as is often the case with rebel forces they are likely to be hidden. For example in the battle of Hoth the ground forces landed before any Rebel ships were up in the air. Similarly with the battle of Endor, Rebel strike forces landed on the Forest moon way in advance of the Space battle.

So it's more than reasonable that both can occurs simultaneously

The combat sequence is attempting to emulate the endings of RotJ and EP1 where you have a ground battle occurring simultaneously with a space battle. Plus, in Star Wars you can't really prevent a ship from landing troops as they just come out of hyperspace right over the surface and drop their troops before you even know they're there.

Having to win a space battle before landing troops would have a strange effect on the Rebel base if the Empire shows up with a weak space force (say, just one of the little carrier things). They only reveal their base when troops land, but in revealing it, they might also reveal enough space ships to prevent the landing in the first place.