I'm not sure on how you're planning on structuring Nassau, but it is significantly different in the real world than on VATSIM. The Nassau FIR on VATSIM (SFC-600) is covered by the Atlantic, Caribbean, and Ocean Areas.
The Miami ARTCC actually does extend all the way to San Juan. I'm not sure why it was separated on VATSIM into Centers. "Miami Oceanic" is just another area of Miami Center.
I would recommend putting Piarco, Curacao, Santo Domingo, Port Au Prince, Havana, Kingston, and Nassau together, leaving San Juan as part of the US division.