How to fix objects that show up black in "Lumen Scene" view mode? How to enable nanite for FBX?

Another question related to glitchy Lumen…do I need to break my barn mesh into simpler meshes? I keep hearing/reading that Lumen can’t handle complex meshes. Maybe the barn pieces are too complex?

Here are the stats on the meshes in the barn already:

  1. Walls and windows are a single mesh
  2. Top layer of roof is single mesh
  3. Bottom layer of roof is single mesh
  4. Weather vane is single mesh

For all meshes, there are quads and triangles only, with no manifold geometry. I made decent UV maps for all of them. All of them are continuous meshes except the weather vane. It has several non-intersecting pieces that have been made into one mesh using the “combine” function.