ACTUALLY...
One reason I prefer rendering at 3840x2160 is post-work. Specifically when working with the clone tool, brushes, etc. to remove glitches and flaws. The higher pixel resolution helps if you are doing 'fine work' in this regard, with a smoother result after downsizing. And yeah, I've had a few renders (without post-work) where a trained eye could see the difference in roughness between the 1920x1080 and the downsized 3840x2160. This particularly comes into play when you do fewer iterations at higher resolution vs. more iterations at the lower one, especially in less than ideal lighting situations. It's subtle, but noticeable for those that look for such things.
But then, I'm not trying to generate lots of renders on any given day, so I can afford to be a bit indulgent with my render sizes.
THAT BEING SAID, 90%+ of people won't notice the difference between 'native' and 'downsized' renders, so it all comes down to if YOU are happy with it or not. Remember that people are mostly interested in the story, not how technically perfect each render is (although the renders should still be titillating). As mentioned above by other posters, a number of people go the Honey Select route, and their games are doing rather well. Some people actually prefer that style over 'realistic' Daz models, because it looks a bit more like 'storybook style'.
That's not to say the Daz doesn't have special shaders if you wanted to do 'cartoon/anime' style renders (these do exist), and some people do use these. My point is that, choose the resolution that you are happy with, that balances your render times/workload vs. the quality you want. Lower resolutions generally render faster, which is perfectly fine, especially if you plan on doing a LOT of renders.
There are a few games that use renders at lower than 1080p resolution (1280x720, etc.).
Finally, you can always go with the 'well, when I win the Patreon lottery, then I can afford that quad GPU system and do really nice renders, but in the meantime, yeah this'll be good enough for my story' strategy. I've seen several game designers do this, going back and reworking and even upsizing renders for past chapters after they were able to afford a major hardware upgrade.
Food for thought. Hope it helps!