I think she's actually the Duchess of Sussex now
Don't think she'll take his last name of Wales - apparently that is what he was called in the military - Harry Wales (I think).
This is from Wales online:
"Once married, Meghan will sign as Meghan, no last name. Just as Harry signs as Harry. Royals use only a first name."
I just checked and I may be wrong about the surname Wales. It might be Windsor-Mountbatten?
On this website from Wales they are calling her "Her Royal Highness the Duchess of Sussex".
I think you are right.....