Rithe fwom your gwave!
So, here's the new system. We're gonna do this GalCiv 2 style.
Instead of armies of different unit types, an army is measured only by its military strength. The unit composition in fluff terms can be anything, tanks, planes, soldiers, dirigibles, batmobiles but the only stat measured is Strength.
To gain Strength, build military bases on a planet's surface. For each you gain 500 strength capacity, that fills up a rate of 200 per base per turn. You can then load that Strength into a ship with Transport Modules - each holds 250 strength.
As for battles, whoever has more Strength wins, with the difference between the two forces' strengths left over to the victor. Whoever's left in control of the planet slowly recovers their Strength using that planet's bases, whether held or captured, at the normal rate.
Different tactics might alter strength ratings at a cost, but we'll specify all that later.