He is not breaking any of the laws of Florida. Charlie Crist ran for the Senate while governor of Florida and didn’t have to resign. But regardless of that, even at the most strict reading of the law, it does not apply until he “qualifies” as a candidate for another office that would run concurrent with his current office. The earliest that would apply is when/if he files paperwork with the FEC to form an exploratory committee. A more liberal reading could be that he doesn’t actually qualify as a candidate until he receives the nomination of the party. In any case, most of the things Desantis is doing are part of his job as governor and are common activities for most governors whether or not they have presidential aspirations.
Rick Scott ran for Senate when he was the incumbent Governor.