In a past life, I led a team that built an octree mesh LOD system that ended up tesselating all of Earth from “satellite height” down to “walking on the surface.”
Building the strips that match up different LOD levels at the edges of the cuts was a big challenge, especially because we modeled in double precision but rendered in single precision with local camera offset (really, the only way you can do it.) The main challenge here is to avoid “sparkles” where the pieces fit together.
If your mesh is less than a kilometer in size, you can just model it all in single precision, it’ll be fine. Cut it into may pieces, and export them as an “array” of meshes. It may be that this is all you need to do, because collision detection will then only collide against some of the closer chunks, rather than the full chunk. However, if that’s not enough, you can then also manually model multiple LODs, making sure they have vertices that match up at the edges (so this makes it harder to get really strong reduction, because the edges need to be tesselated to the highest level.) This will work without changing the Unreal Engine at all.
Once thing we ended up building to solve that was “strips” that bound two LODs together, and filled in the gaps and hooks them up. We tried to call these “hookers” but as it was for a military contract, that ended up getting dropped from the documentation If you want to write your own C++, maybe that’s a way to go? It was a fair amount of work to get it all to work out, but that’s partially because we had 20+ LOD levels, we had to make materials match up, we had to stream the different levels from spinny disks, and the graphics card of the time was something like a Geforce 4 …