Instead of just throwing more and more raiders at a base, have different types of events show up. You guys are constantly nerfing defenses so that raids can't be cheesed. So you're worried about stopping players from doing what works. What you're not thinking about is how raids can be changed so that they're more dynamic and players need to invest in defense strategies that are different and dynamic to respond to different kinds of threats.
Instead of asking "What do I nerf next?" - you need to be asking what can be different about raids, enemy units, equipment, etc.
Maybe there's an alert that a suicide bomber has been spotted, he's going to touch a wall and then explode. Maybe flag that enemy so it's easy to see.
Mid-raid events: 20 tribals show up and attack from a different direction.
Two saboteurs have dropped into a high value room and are going to begin destroying high value objects or maybe they'll just steal something and then mine o...