If all you want to do is roundtrip MIA-GIG then it may well be cheaper to do this on a separate ticket than pay the difference between a 3-continent and 4-continent OWE.

On the itinerary you propose (excluding MIA-GIG-MIA) you could have three more segments (but no more stopovers) in NA to maximise the miles. One of these segments can be a non-stop transcon.

You could also have 2 more segments in Asia (with as many stopovers as you like - stopovers are only limited in the continent of origin), and two more in Europe.