Don't forget that most leave in the winter and head elsewhere. I would think that late September or October would be a good time to go. Hang out for a month and then hop on a boat in November to Belize (north) or Roatan (east). Then fly back from where ever you end up. The Rio Dulce is in the middle of one of the best cruising areas in the world.

Photo, the Admiral free diving in Roatan