I think some of the following ideas might give the ground battle system a little boost.


1. veried manpower cap on both attacking and defencing sides, based on each planet's size and type in the system. 

For example, gas planets have fewer fighting space for the both side. On the other hand,  the huge planet might have too many attacking point making players harder to protect them, so they might coming to a second thought about colonizing a system with larger size planets. 


2. rework some "not quite useful" military system improvement. let them not only able to increase defencing side's manpower cap,but also decrease attacking side's manpower cap(like chipping off their standing ground), creating a unblance battlefleid at the starting point of the goundbattle session. it might make player willing to go for bombard tactic more often.


3. design some unique improvement that can partial nutrify certain type of army(maybe 'planetary trench' can put aircraft out of ground battle), so the ground battle can become

 a little psy game(or pure random guessing, i don't know). in that case, the attacking side need to pay attention on the fight, and quirkly change its army compositon.


New tactics or new mechanic for ground battle are not easy to make, best wish to the dev team.