I think a sterling silver belt buckle is pretty much always OK
I think mixing and matching different levels of formality can be a good look. For example wearing everything you described to a business meeting could risk looking a bit too cartoonish, like you want to be the "chairman of the board" out of central casting. I often wear a Hermes tie with a 3-button sack suit and OCBD, or a french cuff shirt with blazer and no tie. When done right it can look "casually elegant" without being either too casual or too elegant.If you mean one of those sterling buckles you use with a strap, I wouldn't, but it isn't because of a clashing metals issue. It is because I find those belts sort of "business dressy," the sort of thing you wear into the board room with a subtly chalk striped navy suit, well polished black cap toes, a broadcloth shirt with a spread collar and French cuffs, a sophisticated tie such as an Hermes, and a thin gold watch like a Piaget or a Breguet. I like blazers with blue or pink OCBDs, surcingle belts, and bright repp stripes. I find blazers very comfy and casual while, ironically, remaining dressy enough to meet grandmother for lunch In some snooty tea room. I am much more bothered by mixing levels of formality than I am mixing metals. My plain gold wedding ring, never removed in 47 years, will just have to clash with platinum studs.