Ok so I finally found an NPR that was actually a challenge to take down. Single system with no colonies but they were at least 2 tech levels ahead of me on weapons yet had slower ships overall. As I'd pounded the previous NPR to scrap at long range with missiles I expected to do the same here. I imagine my face was very like that of the Peep Admiral in Weber's books when the Manties used MDMs for the first time when I detected missiles incoming at a range of at least 50 million km more than I could throw them. Took heavy losses wiping out their combat fleet and only won through having far superior numbers. (Had to throw Home Fleet at them though, I'd have been in trouble if they'd gotten past me.)
So enemy fleet destroyed I drift the fleet up to maximum missile range of the planet and call up my Bombardment Cruisers. A corvette scout sent in found the huge amount of military contacts on the planet so I opened up with a conservative 5 salvoes of 25 warhead 10 missiles, thinking the population would cave. They didn't. So another 5 salvoes. Still no surrender. Emptied the magazines then in frustration (leaving 1 missile in each cruiser due to my inept design of their magazines initially.) Massive explosions all around but still no surrender. Fired off the last 5 missiles on the basis that I may as well as I had them there, and ordered the main fleet back to the Homeworld leaving a blockade picket behind. Lo and behold the population surrenders as the last 5 missiles detonate.
Excellent I thought. a 1500 million world to add to the empire . . . . except radiation stands at 15000 and the dust is so thick the world has become uninhabitable. I'm beginning to think the resources invested in the Bombardment Cruisers would have been better spent on faster troop transports and more assault divisions.
Ah well, at least i can turn the planet into an irradiated auto mining colony.