This is exactly why a significant number of modern raytracing titles have animated character’s reflections in screen space only.
Going back to my earlier feedback, we probably need a way to flag shadow casters as screen space shadowing only. Won’t work for every case but it should be ideal for certain things like foliage and ground scatter which often needs to move and are small enough to be represented well enough in screen space only.
In your case, I simply would not have the fan blade casting RT shadows. If you need the appearance of a light source shining through a fan to cast a moving shadow, I would try using a light function. I think this should eliminate the overhead of updating the BVH. Then you can have a mask in the shape of the fan blade.
I would probably make a distance field alpha mask, as then you should easily be able to fake soft shadows based on distance by controlling the range of a smooth step.
Perhaps you can switch the closest fan to true RT shadows and have light function proxies for farther ones if you really need it.