Raiding is pretty easy currently. You beeline straight for Kazeron, smash the star fortress and 1, maybe 2 fleets guarding it, then raid it to your heart's content. It would be nice, if gathering enough marines wasn't the hardest part of raiding. Factions' mighty castles are more like straw huts in practice, no siege required, you just go and torch them.
So right now, one can go around survey, salvage, saturation bomb, and raid places with nearly basically reckless abandon. This trivializes Remnant systems as long as your fleet is faster than theirs. And also trivializes attacking factions (once the battlestation is destroyed). As I see things, any such actions, should kick the player back into the campaign map for a moment once started and start ticking down a progress bar giving a chance for enemy fleets to intercept the player and prevent them from doing the action! And this ties into the next point which is a much larger scope suggestion but would do so much for the game.
One good thing is that it provides context as to why sometimes a distant fleet prevents you from salvaging. If there was a bar to fill while you scavenge and it wouldn't get filled before the fleet gets to you, you have some indication as to how close is too close. Those bars should be instantly filled if there's no (potentially) hostile fleet within a reasonable range, though.
I agree with the rest of that post as well, but I don't have much to add.
The least I would want to see done about raiding being trivial is that some colonies have defence fleets and that Military Base and High Command should pull the fleet size bonus and fleet numbers from the biggest same faction colony in the system.
Defence fleets should almost never leave the station's reinforcement range and scale with home colony's income or size. They should be guaranteed for colonies with colony items, with heavy industries or with size of 7 or bigger. They are also a good place to sprinkle some s-mods into non-event fleets. If raiding stops being instantaneous, those fleets could also roam somewhat.
Military Base and the upgrade should scale, as mentioned, because while "big civilian colony with small military colony" set up is flavourful, but a bad decision from gameplay standpoint. It's not the small colony that has to be protected, it's the big one, and the big one doesn't have suitable defences. Military Base and High Command getting the colony size perks from the biggest same faction colony would solve this issue, as well as create an opportunity to weaken your raiding target in advance, by first disrupting the smaller military colony.