There is no universe in which you should need a 50 million triangle moon. Even with Nanite compression, that’s going to take up way more system memory than it could possibly justify.
Decimate it to 10% of that. Bake out a displacement map and use Nanite tesselation and displacement if you really want that level of detail.
Really, you just use a smooth sphere and Nanite tesselation and height maps for all your planets (though 4 billion unique height maps is still going to be an impossibly large data set.)
From some of your other posts it seems like you’re trying to make something like Elite: Dangerous meets Kerbal Space Program. Both of which use heavily procedural systems to generate their planets. You’ve got a lot of work ahead of you.