With the removal of PDC there is a few extra things I like to see to counter the bombardment by missiles.
By the looks of things a navy could sit out of range of the planet and bombard the forces on the planet with size 1 missiles, which out any strike back which the PDC, with there huge armour ability had the protect and the ability to fire back. I think Orbital starbases are hugely vulnerable. Perhaps you can add to the building slots like factories and so forth "Defensive platforms", which are purely phalanx style defensive guns which shoot down inbound missiles, or if you do not go that way perhaps make some of these mega robots with defensive abilities such as they are able to effectively mount Anti missile defensive, maybe some of these Robot could mount shield generators.
I think the planet need a way to counter the AI in missile bombardment, the AI currently does this, as it is come within missile range it then bombard the planet. I like the idea like in Starship troopers where a Navy flies by and drops pods down to the planet and then moves away with possible losses. Planet capture should be a risky procedure, until there is a reduction in the planet defenses. I would like to see beam or railgun fighters useful in a ground role. It would give a reason to develop them.
I would of liked PDC to stay, because they are really just a static fortress, with the inclusion of all the other things. I did not find them exclusive of each other. I would like to see options in the ground combat, in what your attack goals are. EG reduction of orbital defences, reduction of industry, destroy stockpiles.
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Any chance planetary water value can influence certain effectiveness of units, such as if it a higher water content world it is harder for larger units, due to marshlands, forests for higher oxygen content planets. Where as desert worlds or lower oxygen content worlds hinder standard light troop units. etc. That way there is no one size fits all planetary invasions.
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Wow I cannot wait for the planetary invasion updates even these minor changes you suggested makes me want to not glass planets.