Trying to make the most out of my 3 model limitation...
That looks great!!! Is "Receive shadows from surfaces" checked under the cloud's quality tab? If not, you should try this. It will take longer to render, but it's maybe worth it.
But maybe it will look better the way it is. Somehow more mysterious...
I tried ticking the setting as you suggested, I have 2 clouds under my atmosphere tab a cumulus layer to create fog in the general atmosphere and a cloud layer v2 for another lower thicker fog.
Ticking both clouds created a much darker scene which honestly shows the lack of detail to the floor which is not ideal...
(https://i.imgur.com/ypkEEKw.png)
Ticking only the cumulus layer made the top half darker while retaining the thick fog at the bottom which i think its a good alternative, it certainly helps to bring a more horror vibe that I initially was aiming for.
(https://i.imgur.com/gNxr4cP.png)
A good example for artistic freedom. The last one looks indeed best imho.
Cool stuff.