I found lighting figures with the background very difficult as Poser shows only a restricted image of the background in preview,  so I have tried another way.   Render the scenes with the selected background and use an AI background remover to mask the image.  Replace the background with the image but as a .jpg suitable adjusted, which should get you back to square one but now you have a layer of each which can be moved and adjusted accordingly.   It will only works for figures and I suspect the same could be done with two renders, one for the figure and one for the background.  Anyway this is what I achieved in about thirty minutes using an existing render.

 

 

I use Poser 13 on Windows 11 - For Scene set up I use a Geekcom A5 -  Ryzen 9 5900HX, with 64 gig ram and 3 TB  storage, mini PC with final rendering done on normal sized desktop using an AMD Ryzen Threadipper 1950X CPU, Corsair Hydro H100i CPU cooler, 3XS EVGA GTX 1080i SC with 11g Ram, 4 X 16gig Corsair DDR4 Ram and a Corsair RM 100 PSU .   The desktop is in a remote location with rendering done via Queue Manager which gives me a clearer desktop and quieter computer room.