To answer the question you asked. No, changing rim size does not affect manufacturer warranty.
Changing rubber size gets a little iffy.
Downsizing from 24 inch to 20 inch, but keeping the same relative diameter of tire won’t even make Ford blink. They might give you hassle if you upsize the rubber, but normally they don’t even bother with that. It’s hard to prove that larger tires cause issues. The Magnusson – moss act gives you a lot of protection in that area.
My ‘24 king Ranch has the 22 inch rims. I personally think they ride really nicely. Especially compared to my ex excursion or my raptor. Pricewise, the 22’s aren’t exactly cheap. So 20s may be the way to go for best ride (softness) and price factor.