Very much depends on depreciation, and that in turn depends very much on the make. Some cars depreciate like a chocolate teapot as soon as they leave the factory, some hold on, and some models even increase in value for a while. No easy answer

No.

Or more specifically it exists but you'll never know what it is.

I am guessing by optimum frequency you mean resale value within your 0 to 5 (or 6) years of ownership? If so this depends on entirely what car and what mileage you are doing. Nowadays, with care, most pre 6 year old cars shouldn't be suffering major problems if cared for. It might come with a free extended warranty period however?

Other than that, cost wise I imagine keep the car for your alloted 5/6 years, using the period to save for the next car fund, and adding on the final sale/trade in value of your current car.

People will always disagree about this one. Personally I think that you either change after max. 3 years / 40,000 miles or so - i.e. while you can still get a good price for your car, or else run it into the ground.

There is a better case for the second alternative the lower your annual mileage.