I've reviewed my work with something bigger in order to get more freedom. So I've started all over with a map at 2048 x 2048 with 300 dpi.
Below, the basis:
 

The main difficulty here was to represent the cliffs in the South part of the map, something I finally solved by duplicating the land mass layer, then by applying an emboss effect and finally by associating a layer mask to hide the unnecessary parts. Last step was to use an oil paint filter.

I'm currently working on the rivers and mountains.