Optimizing large interior with many static meshes

I built a very large interior with thousands of modular static meshes and props (also static meshes).

The whole level consists of static meshes and lights.strong text

The performance drop as of late is very concerning after reaching an fps of about 2 fps!! Whaaaat!

So, here is my question… what steps would I follow to optimise the rendering?
I did a little research about instanced static meshes, precomputed visibility, lod, hlod, etc.

What would be the fastest and most effective way to optimize this scene without merging?

It is rather important to keep the modularity as is.

f possible, please provide me with some steps on how to do that as well. Should I go with precomputed visibility with nanite?

When would static light be more effective than stationary and movable lights, exactly?

I also use raytracing, lumen and shadow maps.