Most of the devs on this site use DAZ3D to create their renders, the software itself is free but the assets are not. You can find so many tutorials on youtube, just type DAZ3D tutorial on YT you'll find thousands of them. Assets may not be free but you can also download assets on this site because of course, this is a pirate site. You may need to find the most powerful rig you can to be able to make a full game, cause while using Daz does not require your pc spec that much, rendering images do.
Generally, what I saw in a VN's single update got around 200-600 renders depending on the game (some may take more than that), so if your one image takes like 1-2 hours to render, to render a hundred images would be a nightmare. There're many techniques to reduce the render times but in the end, the real game-changer is still your rig.
I'm a newbie myself so I can't say much more than that, all I know is very surface information. You need to wait for other experienced daz3d users or devs themselves to answer your question further. But, I hope my answer would be at least useful to you.