Least of the engine problems… but a problem nonetheless.
If your final render is static I suggest making a manual merge in photoshop between the 2 different renders.
Even just setting the denoised image on top at a 50% transparency should look better and preserve enough details.
Short of that, I havent found much. The more rays you have the lesser your return on investment is.
There is a quality jump between the default settings and its double, but increasing past that gets less and less quality for rendering cost. To the point increasing does nothing.
It seems the core issue is due to the shadows and how the default engine settings are computing them…