I saw some vids before of your work and it’s most impressive + you have Steam Green Light.
Yummy-Vegetables appears to be the expert here(and several other posts of his I’ve read) but fwiw using Landscape I’ve come to the conclusion it’s best to work backwards. Cutting, shaping etc, and finally, but 1, texturing. ‘But 1’ is lastly allocating the Physical Material.
My invisible Landscape’s Physical Materials don’t work now because I did more shaping. It needs replacing, probably with a mesh. Lesson for me then is once the Landscape’s done, it’s done - no more touchee.
But my Landscapes are relatively small, tiny even, compared to your massive Landscapes. Also as you say UDK’s excellent Landscape LOD setup would be difficult to emulate, particularly if players can move in all different directions. My Landscape takes players in a linear direction; roads thru the countryside to a village or a complex.
And, vertex painting my meshes is bit of a pain, but on your kinda scale, a massive pain.
It’s plusses minuses. I definitely prefer working with Landscapes and I think I’ll give it another try, but as I say, ‘backwards.’ Sorry I couldn’t be of more help here.