As much as I’d like him to be wss going into the final arc, because Luffy should be PK by then, I do feel an admiral fight would be underwhelming if it came after Mihawk. Unless Zoro doesn’t look like an underdog in the admiral fight and Gandhi afterwards happens to have a hidden strength that rivals Ryuma(the aged down/Bonney theory) but even w that route Gandhi’s blade isn’t even black, his final opponent should have a black blade. The best course of action I think is Zoro fighting Mihawk after the war but I really don’t know how Oda will execute it, usually Luffy gets the last duel. UNLESS Mihawk involves himself in the final war somehow as an antagonist. As for Sanji there isn’t really any Mihawk class opponent for him to duel in the end his character arc doesn’t need that, he’ll get Kizaru though imo. As you said if Zoro gets a tough fight Sanji usually gets a somewhat comparable one but in this case Mihawk will be the exception.