Planetary defenses would deter invasion forces though. Maybe it would damage them as they are landing on the planet. I feel like planetary battles need some work, if it got some, it could end up being much more interesting, dynamic, and engaging. One change that I think is needed is to make light, and limited bombardment only destroy a portion of the planetary fortifications, not all. With light, you only target identified military fortifications to avoid civilian casualties, some fortifications are bound to survive.
It is also problematic that you can just invade with a whole stack of hundreds of armies at the same time. It makes trying to defend pointless. All the features involving planetary defense are obsolete, that is no fun. Maybe only 12 armies (as many as the maximum garrisoned armies) should be able to be attacking at one given time, giving some balance, and reason to the defense vs offense situation. If your transport fleet has more armies, the rest would reinforce as the ones on the ground die. Introducing this change wouldn't cause more micromanagement, but it would make planetary battles and strategies into something intriguing. This might make generals and attachments useful as well to get an edge. You could then get more creative with generals traits, armies, and attachments. It keeps things simple, but makes them way better. There is a lot of room for improvement currently, but that doesn't mean you have to over-complicate things.
Also, I noticed that there is a part of the UI that shows the defending fleets vs the attacking fleets, but currently, it is impossible for to fleets to meet in battle while in orbit of a planet as they would engage before that happened. Maybe that would work if the game was turn-based. I don't know if this is left from when the game was in a different developmental state. I don't know if there is a way to make it work now, but if you did, I'd be interested in fleets battling in orbit as well as performing ground support. But fleets are a bit too large relative to the planets, visually speaking. Eh, I got a bit sidetracked.
I just think that the game would be more fun if offensive strategies were balanced vs defensive strategies. Right now, you can just forget about trying to defend except in early game.