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VMblast I may be able to help you a bit with that, just so you know, I've had quite a bit of success importing terrain sculpts into Godot from Blender. What I do is I get a regular plane then subdivide it enough for things to be smoothed out nicely and go to town with the sculpting tools, what I found interesting is the texture paint seems to import fine as well. I haven't done anything particularly fancy as of yet but I have managed to make a basic island that looks alright, this is what I came up with, followed a water shader tutorial and I need to tweak the meshes for that to work all cleanly but that came out nicely too.
It may even be worth doing some terrain generation in Blender itself and then exporting the results, sculpting I feel allows for a lot more control.