I don't understand your phrasing. Are you calling Sabo yonko first mate level or Burgess? We've only seen Sabo fight one Yonko crew member so how can Sabo be Yonkou FM level if The man he fought, Burgess, was not a Yonko FM? Burgess has more likelihood of being a first mate over Jack as of now because we don't know anything about Jack or when he joined Kaidou while we do know Burgess was an original BB Pirate. We don't know if either are FM so they are equally as likely. FM doesn't mean strongest it means literally first mate.