As you said, good lighting is crucial. But it's the hardest part to master IMO
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, especially indoor scene with low light.
I'm not a specialist on lighting, so I will not tell much on this topic to avoid saying incorrect advise.
Maybe some other can give you some hints.
I will just advise you to look for 3-points lighting technique on internet.
What I also think add realism is to have a detailed / coherent environment (ex: add few carpet, towels, ... , lower the bath water level, dot not place all items @ 0/90/180/270° angle )
For the Ren'Py, I think it is doable, but I don't know which part of the game code needs to be modified. And since I only have a 1080p screen, I didn't bother investigating this functionality. Maybe check with the latest version of AWAM if this is something that is implemented.
Ps: your Sophia looks nice, continue like this