I don't know if it's an ironic thread or not. Btw, Oda tends to repeat himself. The strongest example I can do is how an arc is repeated with the same events over and over (the crew goes on an island, they found the bad guy, the bad guy win for the first time, there is the "time bomb" deus ex machina, the crew win and then escape from the island). Of course it's not always like this, there are exceptions or variations to the formula. But considering this I can see the similarities between Sanji and Shanks but I would not consider them important or like there is some deep meaning sincerely.