Well, one issue is that it becomes easier for countries to actually get nukes once the whole technology is known. One needs to start with less uranium and needs to refine it less.

Regarding the Curies, while that it is true, it might be that people would have noticed radioactivity earlier. And more U-235 around means more radium around also. But I agree that this probably wouldn't have a substantial impact on when things would be discovered. Given how long a gap there was between that initial discovery and the idea of an atomic bomb, even if it did speed things up it is unlikely to have impacted the development of nuclear weapons that mcuh.

Your points about profileration and effectiveness seems to both be strong. Overall, this conversation makes me move my view in the other direction. That is, this seems to be not just not a strong filtration candidate, the increased ease of energy access argument seems to if anything push things in the other direction. Overall, this suggests that as far as presence of U-235 is concerned, civilizations that arise on comparatively young planets should have less not more filtration. This is worrisome.