In my opinion, it would be best if the battle was divided into three phases, with the player determining how to go about each one. An evoloution of the Galciv 2's system.
Said phases being orbital, landing, and ground combat. Players pick an offensive/defensive option for each one.
Orbital phase would cover things like orbital bombardment of structures (Attempt to destroy random structures), clearing a path for landers (Refuse to act during this phase and risk any ships in the fleet that are not transports for increased roll in the next phase), shooting down landers, defending against orbital bombardments, tactical nukes, and various other pre-ground options.
Landing phase would be where you determine how your troops get down to the planet. Drop-pods (Very low risk of losing infantry, less combat strength), landing craft (High risk of losing, more combat strength), Decoys (Assured to lose some troops to get others through, normal combat strength), and so on. the defender has to try and guess your method.
Ground phase covers what your soldiers do on the ground, be it to set sleeper agents (Increase strength of next invasion), attempt to take over. Attempt as much damage as possible before retreating, or literally destroy the planet due to core detonation.
Each phase consists of both sides picking their options in secret, showing at the same time, and rolling.
