Honestly I think what would really make ground combat better is some kind of ground battle other than just “battle” as we have it now. Maybe allow strongholds to force a siege, which would drastically reduce the damage that a garrisoned army would take while also reducing the damage dealt, so that a fortress world is still takeable but would be a much longer fight, possibly allowing the planet to be relieved (something like a stronghold could support one army garrisoned in it with appropriate bonuses, with more armies inside as it upgrades). It would also be great if ships overhead were involved, perhaps to include having ships with hangers on them be able to use those fighters to support the ground forces. Likewise, a planet could have more defensive buildings, such as hangers which could support fighters to either fight on the ground or to attack ships overhead, or anti-orbital weaponry to fire back at enemy ships in orbit.