My thoughts on fixing ground combat involve changing the way planetary bombardment works entirely. First planets greater than colonies have 1 missile silo per unlocked building per capital level, these planets will shoot at fleets engaging in bombardment. Second only missiles and Large and larger weapons can actually damage a planet, smaller weapons would either dissipate or disintegrate in a planets atmosphere. Third a new "bombardment" stance would be created called blockade that would shield the fleet from a planet's weapons but also prevent the fleet from inflicting damage to the planet. The only affect of blockaded stance would be to prevent the planet's resources reaching the empire and vice versa, potentially starving out a planet over very long periods. Finally cruisers would be modified to have configurations that make them better at attacking planets than any other ship type, possibly with supplementary defensive destroyers.
All of this would make it hard to actually take a planet through bombardment alone unless it was only a colony. You'd have to dedicate significant resources to bombardment fleets, or build up space marines to bring the battle to the ground before the armies have been defeated from above.
