Since POTUS claims mail-in voting is corrupt, what should he do rather than request a mail-in ballot for the Florida primary?

Preferred that he vote in person but you are being a little disingenuous here. Absentee voting is not the same as mail in voting. If you vote via absentee ballot you have to request a ballot from your district. The district is then responsible to check that you are indeed an eligible voter and sent you a ballot for you to fill out and return. Mail in voting is the district sending everyone on their voters registry a ballot and having the ballots returned by mail. Vote by mail is way too open to fraud since anyone can fill out your ballot and send it in. You have no recourse if you lose your ballot to a professional ballot marker. You just voted the way the professional ballot marker marked your ballot. You never even saw the ballot to begin with.