How to export level (map) as an external (non-bundled) asset?

I would like to build an FPS game where the levels (maps) are external (not bundled with the core game) assets. In other words a player should be able to download/install the original core game and optionally download additional levels (possibly community content hosted somewhere else) without replacing the entire game.

The separate levels assets should still take advantage of

• Unique textures/models for each level's terrain, decor, skybox, etc.

• Use Unity's build-in terrain system with trees, grass, wind system, etc.

• Occlusion culling

• Lightmap data (baked GI, reflection probes, etc)

• Include scripts (or configuration) that modify the game rules

How can I setup a workflow that will allow me to export all the above mentioned data separately from the rest of the project into some sort of format that can be imported by the final product at runtime?