Maybe it’s dark because there’s no lights? Looks kinda realistic given that there’s no light sources
Lights in RDR3 are #typ’s CBaseArchetypeDef extensions called CExtensionDefLightEffect nowadays.
One of the vertex color channels can affect emissivity, but don’t expect such lighting to be beauty.
Occlusion (boxOccluders and occludeModels in #map files) is not related to processing global illumination afaik.