I was thinking what's the true nature & purpose of the ground combat and I think I will redo the system from scratch...
Ground combat, in its core, is about slowing down the conquest (after you defeated the fleet protecting the planet you need to send ground troops and wait til they do their job). A planet should have "conquest meter", once it reaches 100% the planet ownership changes. The meter should progress based on difference betwen defending and attacking infantry. Also the fleet should probably move the meter on its own (bombardment) so eventually a fleet could take over a planet without ground troops, but it would take too long to rely on it alone (like 5% per turn).
The defensive troops (regular infantry) are divided between all your planets (based on planetary defensive priority). Similarly offensive troops (stormtroopers) should be divided equally between all planets when you have invasion in progress (so if you have 100 storm troopers and attacked 5 planets on each 20 stormtroopers would be deployed). In addition, since you can deploy stormtroopers on your own planets as an additional defence, these count too (it's a trivial number of stormtroopers / number of planets that requested their presence mechanic).
This system has nice properties, it makes the player limit the number of simultaneous conquests (or these might progress too slow) and reinforcing planets has an opportunity cost (you can't use these troops for conquest).
Now, I wonder, should I complicate it more? Like non instant deployment on a planet? Or other issues you can see here?
(I would hate to reimplement it once again :))