Raiding should be valid to take down the big bads on a planet, because if you take that away, then you may as well just load all your bombers on a transport and go hog wild and just rebuild them after the fact. What I do agree with is the idea that OMDs need to be much better at, say, completely demolishing champions and golems and FS ships. Maybe make em better at killing other starships, but raiding's always kind of the go-to answer when you look at a planet and think "Yeah I just need to hop in and kill something really quickly without it being an all-out war." Take that away and you just pile up bombers to knock down OMDs. Where's the strategy in that?
...in fact, make OMDs more appealing to capture. That's option 2. The more appealing OMDs are to a player, the more likely that they're going to actually attack and take the planet without actually destroying the OMD... meaning that it can just do all of its damage while the attack is on going, and the player has to weigh that with the idea that "Hey this will be useful later." I can already think of one way to refute this, and my response is "10/10 games are not what the game is balanced around. On 7, you can take enough planets to afford grabbing the occasional Ion/OMD." What should the exact solution be? I have no idea. I'll hand that over to the experts.