Agreed with what Dr PinkCake said. Even with dual 1080s, and I render at the resolution you are at regularly, scenes can take a while if they are very complex (read: lots of objects/detailed textures). I've jumped on the HDRI bandwagon for lighting, which seems to help, but that's less useful for indoor situations.
If you are doing the Renpy 'overlay character over scene' thing, then investing some time in your background images (if you only have a limited number of them) may be worthwhile, but with your hardware specs, don't feel bad if you have to settle for slightly less quality by rendering at 1080p instead of 2160p, in the interest of render time management. And, as pointed out, 720p is used in a lot of games, and does just fine in most situations.
I'm not a lighting expert, but a few people seem to really like ghost lighting. Lack of lighting in a Daz Iray scene can really push up the render times. Not necessarily making things brighter, but amplifying things a bit to help better define objects in the scene. I'm not explaining it the best, but I'm sure others can give you some good tips along these lines.