I'm not sure if this would help any, but how do the British usually refer to the royal family? The descendants of Elizabeth II, on legal documents and such, occasionally use the personal surname Mountbatten-Windsor - for instance, Harry and Meghan's son Archie has that as his surname. But how well known is that? Is using the name "William Windsor" a clear sign of an overseas person (an American) being a big fan of the royal family? (Assuming that they're putting together "William" and "House of Windsor" to get "William Windsor"). Or would someone living in Britain might think that as well?