I think The Wales children were known as George, Charlotte and Louis Cambridge. Now the ware known by the surname Wales just as Harry was known as Harry Wales
Take a look at the discussion earlier in the thread.
Using 'surnames' based on peerage titles is a convention in Britain, but has no legal status outside of Britain. As you know, Harry and Meghan found freedom from Britain. The 'Sussex' name is not recognised in California. Their family name is Mountbatten Windsor, as is on the children's birth certs. It is not 'Sussex'.
Obviously being associated with a British peerage title is incredibly important for Meghan, hence her insistence on using a name that is not actually her legal surname.
the King (ridiculous title in this day and age)
Well, he is King. That is is his constitutional role, as defined by law.
How is it any more 'ridiculous' than a jam seller in California going by the courtesy British peerage title of 'Duchess of Sussex'? Complete with 'office' and little doormat with her coat of arms?