Well i'll give you my perspective, time of day is a tricky one beacuse it depends on what i'm out to achieve and personal preference. I think almost anytime can look good and realistic, differing times on certain tracks can look quite different but again it hard to really say whether one time or times is better for a particular track.
I think preference plays a part in that. I most often prefer the photographic golden hours, sunrise/sunset times. But for exact times well that's diffferent for each track so it would be impossible to quote universal times for you. For instance 8:30am on SSRX looks quite different in brightness and feel from the same time say in Spa or Willow.
But the warm hues of the ealry morning are generally nice.
If you're going for realism, well almost anytime can work but some good editing helps. I think weather can play a bigger part in the look of a shot more so than time. In some tracks 0% weather can look quite realistic (eg. Sazuka, SSRX, Silverstone) and if you want a softer shadow look 25% - 50% can work pretty well too. 100% can work well sometimes but is harder to get looking realistic I find.
But again this all comes down to what you're trying to achieve look-wise. Most of the newer tracks are pretty darn good with their light and textures so that makes a difference.
As for paint I think Solid's usually come out more realistic more often, but metallics aren't far behind since they've been improved in GT6 from GT5. Again some good colour/contrast and tone editing can help here. I think the hardest paint to get to look realistic to me is Polarized/Reflex style paints. That can vary a lot in look based how it interacts with light.
Finally I don't believe there is such a thing as an 'easy setting' that will magically make shots great. This all boils down to what look you are after. Generally for moving shots Panning mode 3 is the most useful and playing with your camera distance from the car itself can vary results a lot. Changing f/ values is quite important too. For close up shots or shots where you want to isolate certian small or part featurers then a lower f/ value is usually better (eg. 2.8 or lower.) And for more distance shots or wide and panoramic shots a higher value is oftern better (eg. 8 or more.) But for a standard distance in which a car mostly fills the frame then the default 4 or even 5.6 works well.
But above all, I believe there is no easy way, it takes patientence and experiementaion to figure what works better and what doesn't. But once you have a feel for how things react with setting changes it doesn't take long to do tweaks. The key is tweaks usually need to be made according to whatever the shot is, there really isn't one setting to rule them all that will work on every shot situation.
I hope that helps. 👍