there are lots of things to try, remove as much unneeded geo as possible, use instances where ever you can, group things so that you can turn them off as you work and concentrate on a different area. If using a GPU renderer, make the textures as small as possible (without losing quality) use greyscale images for channels that require them (rather than converting on the fly in renderer) Look into a LOD workflow. Try a different method to achieve the volumetrics, I rarely do ‘proper’ volumetrics, ill fake it where I can, depth passes are useful things. I’m not sure about RS but in octane, there are many ways to speed up renders, Gi clamp, blur caustics, adaptive sampling etc…
If its a render bottleneck, make sure nothing else is using GPU at the time, same with cpu and ram.
Im sure theres more things that would be of specific help, if i think of any during the day ill get back to you!