I would stay in Miami close to the port as traffic can be crazy on I-95 in the mornings from 7-9:30 am. They are doing construction on the connecting (395) highway that takes you to the port tunnel. Depending on what time your flight comes in from PBI if it’s during rush hour times I’d eat up that way and then head down to Miami once traffic is lighter it should take about an hour and a half to get from PBI to downtown Miami in regular traffic.