Probably want to take the coastal route towards Cartagena, nice airport, can clear customs and fuel is available and then proceed to Rio Negro via the Magdalena. I would consider this option if range is a concern. Definetely those mountains are quite high. A plus for you is that the wx should be improving this time of the year. I do not recall of any major airport either west of Rio Negro where you could clear customs and get Jet A and neither east of Panama/Tocumen, besides David to the North.
Good Luck!!!