These things all tie together to form a cohesive system. Winning the fleet battle should fairly provide the attacker some level of advantage for the upcoming ground invasion. Planetary Shields address bombardment pretty much, you can stack I think up to like 85% resistance for the planets Garrison. The orbital guns prevent the ships from camping the planet for 50 years with impunity. Ground and Naval warfare, are not wholly separate things. One is going to logically heavily influence the other. Ground soldiers cannot logically point their little Space AK's at starships, Large defensive systems like Orbital guns or satellite networks is their method of response, and those would be tied to Garrison size, technology level or whatever. You preserve your chance of winning, by making it impossible to ignore your defensive army, They have to address it to progress.
You create custom army templates in a mirror copy of the fleet management tab. Army Management. "Okay train 3 infantry, 4 tank divisions, 3 fighter squadrons, 6 Battlemechs" etc etc You can do this for offense and defense both. The space combat in the game is already about as hands off as an RTS system gets. The ground combat can still retain that aspect since the game can't pause and there's likely other things going on. You basically copy the naval system, onto land. If you wanna really simplify it. Mechs =Battleships, Tanks = Cruisers, fighters = carriers (Duh), Cyborgs or otherwise Master Chief style Augments= Destroyers and Infantry = Corvettes. You design this into a system that basically plays out like space combat does. the battle starts, and there is visually rendered units fighting it out on a field (if they wanna be really fancy the field is determined by planet type), on the invasion screen (that you can fully choose to ignore ofc) and the outcome is based on all the same factors that determine naval outcome. tech, numbers, and composition etc. You win or lose. If you win, your defense army begins to replenish from the planets population (that are logically hiding in underground fallout bunkers), if you lost mechs or tanks, you gotta que up more, and those build timers can be influenced by devastation level (to help the attacker ofc, they still need a chance). I'm not really sure why ppl think this is so impossible to fix with ground combat. their space battle template is their best foundation to go with.
And to ensure the defender does not lose their Fortress World to Riots due to devastation level, they can build (if your willing to use a slot for it) Underground Fallout Shelters, that give you a planetary decision you can activate that will shelter your Pops and provides them MRE's and water (or whatever your race drinks) and basic amenities, but naturally their not producing anything. The balancer is, this decision will have a massive upkeep cost, that scales with pop size. And to prevent defenders from cheesing the cost by relocating all their pops, the Decision adds a Travel Ban debuff, no Civilian Traffic in or out until devastation is repaired to a particular threshold.
