Trouble with rendering/image mapping in GIMP

[DeductibleTaxes]

Heyo! I'm new to the community, so please advise me if this should be moved to a different forum/sub forum, this seemed to be the most appropriate but I might have missed something. I've been working on a world map for a while now using a mix of methods, mostly using ShortValleyHiker's method though (Including their fantastic brushes). I have a completed height map (Although there are a few things I'm not completely happy with, if you have any advice/suggestions beyond the post's focus please do let me know :) ) that I have run through Wilbur (Again, if there are any mistakes you can see that may have stemmed from a mistake in Wilbur, please feel free to let me know, I have a feeling the very 'stratified' nature of the map did not lend itself to Wilbur completely). I'm currently trying to render the map using gradient mapping, however said mapping seems to come out very 'one-note', I imagine this is in part due to the gradient I created, but also the height map itself. Additionally I cannot seem to get GIMP to isolate the rivers using any selection tools beyond manual selection, so I'm having trouble getting the rivers to reflect on in the final map (Also running even one more round of precipitation erosion seems to get rid of the rivers). Please let me know any suggestions/fixes to the problem. I've attached both the height map before and after an additional round of precipitation erosion and an attempted render (Using the heightmap that actually shows rivers).