Generate UV Lightmap on large causes crash

I’ve created a large cave mesh (to use as my terrain) in Blender. Importing into Unity as well as any adjustments to “import settings” take a very long time. I’ve split the mesh up into multiple meshes in Blender before importing into Unity.

Map Mesh

If I click “Generate Lightmap UVs” and hit apply Unity will hang for about 20-30 minutes and then crash with an error report screen. I’ve also tried just importing one of the subsections of the mesh (about 1/40th of the size), but the same issue occurs.

Is there any way to generate lightmap uvs without using Beast / Unity? Has anyone else had this issue and can possibly help?

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