I like the example you did with the volumetric atmosphere but i understand what you mean about render times. It looks like you have not set Fog in Sky. With a standard atmosphere make sure you have the Fog in Sky Thickness set to 100% and max altitude around 20-30. Then set the fog to around 200 with around 20-30% falloff and gather at low altitudes. Tweak max altitude and falloff to get the exact look you want. This is in V5 Esprit, i guess it's same/similar in Infinite?