For Wilbur i like to use the 32 bit BT format
resave the base map as a 32bit floating point
gmic and Nip2 can do this just fine
use GDAL to convert to bt
Code:
gdal_translate -of bt basemap.tiff WilburInput.bt
run a few Cycles of precipitation based erosion and incise flow
and export to bt
( 32 bit has + and - pixel values )
and use gdal to convert to a 32 bit tiff
and a 8 bit normalized copy of the float
for lower coastal area drop the GAMA to say 0.6
then run through wilbur
shaded using the gimp "bumpmap" shader and a 8 bit copy of the heightmap
.