Nuclear Radiation almost killing people is probably the most problematic, there will be all sorts of long-term damage.
The sci-fi disease at least has the exact properties you want it to have, and the Alien Creature option simply gives you physical damage which comes in your choice of severity. I'd lean toward that over all, but it really depends on what fits the story best.
And might I recommend Royal Road as one of your publishing platforms? Pure fantasy tends to do better than Sci-fi, but at least it is a point of traction. I know a lot of people post across multiple platforms.