One way to solve a problem is to sidestep it. Remember: there is no spoon…
Take yer normal 2k landscape, RVT, etc and just-scale the landscape by 5x whatever times. If you can accept that you will lose some of the fine-grained detail this will work; it’s the same 256 components I had at 2km by 2km but now it’s 5x bigger on a side and in the world. This would be a 10km by 10km area.
Use Heightmesh/Nanite-Landscape deformation at the small-scale for your bumps, rocks, etc. You won’t really need these in the height-information. These details can scale with the UVs of your textures/shaders so they can be independent of the underlying gross-featured landscape, thus you can make all the small bumpy-stuff very dense if you so choose. In other words scaling the landscape doesn’t scale-up the details necessarily, you can claw that back in materials.
You CAN have both worlds if you are flexible.
If you are using the Heightmesh, I would recommend against using World Partition, for the landscape at least. Functionally it does work with the rendering to the RVT, and everything else downstream but as the distant, streamed meshes are less-than what the actual landscape is, you get funky shadows where they are not supposed to be. You can use WP but take the landscape out of it (which if you are using heightmesh should not render landscape in main-pass anyways…).