I really like Carthar's idea of a mechanic that leads to sunsetting the Raider threat. Maybe give them an extra module in their ship called Jump Drive Localizer, such that you have to capture/salvage enough of their ships that you can reverse engineer the technology (it's not in our tech tree), and once you have that, you can research a sub-tech called Jump Drive Localization Blocker, which lets you set up a wide area block against Raiders entering (could work like sector/naval HQs in how many systems it covers).
