Sorry, if the sun or another lightsource is moving it is not possible to use saved irradiance maps or lightcache. You need to calcuate the lighting for every frame.
I wouldn’t say that. It is a very common mistake. Flythough type animation is for moving camera only. Any changes to lighting, geometry or even materials really need the lighting to be calculated for every frame.
If it’s an exterior it shouldn’t take too long to do in any case.