In the Rio Dulce, March and April are hot. Unbelievably hot. Summer is warm and humid like all of the Caribe, fall is almost constant rain like most of the Caribe, and winter can get the big cold fronts like the Bahamas
do, but is pleasant like the Bahamas
The Swan islands are a more fair weather
anchorage - you would not want to be there when cold fronts come through or when it is really honking. Your idea of hugging the West coast
of Cuba and dropping south is a good one, but consider giving up on the Swans if the weather isn't cooperating. Instead just angle over to Guanaja or even Roatan - it will keep you out of the current and the difference in mileage between Swan and Guanaja is only 50nm, so Swan doesn't break up the trip substantially.
But if you have the weather for the Swans, why not? Few cruising boats go there (we haven't yet), but it is a safe place except for bad weather. The underwater life is supposedly pristine.