Since he’s sixth in line to the throne, the blood of his heirs won’t make any difference, barring a catastrophe in which William and his children all die at once.
I don’t think the Windsors are inbred at all. Charles married a commoner twice, Andrew married a commoner, William married a commoner as did Harry. The Queen married into one of the royal houses but she hardly seems inbred to me. Her sister had two marriages, both to commoners. The Windsors themselves, have always been thought as interlopers in England.
She is not the first biracial. Queen Sophie Charlotte was https://aaregistry.org/story/englands-first-black-queen-sophie-charlotte-born/