Huge city, how to split it up for best performance?

I’m building a huge city(about the size of most of manhattan), however there are only 65 townhouses, 53 skyscrapers, 27 industrial buildings and 17 unique “landmark” buildings. except the “landmarks”, all the buildings are repeated over and over, but in a confusing pattern to make it harder to remember each building.
The streets are pretty low poly too.

I’m have pro and I’m using occlusion culling.

how should I group the parts for best performance?

Edit: What I’m really asking for here is modelling help: how large should each group/part be and what should be grouped together. I’m still undecided whether to make each building as a separate mesh or make whole city blocks as one mesh:/

Here’s an example:
51700-howtosplithugecity.png

Take a look to this part of the Manual. It could help you

you can dowload a Combined mesh script asset from the asset store, Personally my favorite cause it automaticly makes it a shared material though its costs money: Unity Asset Store - The Best Assets for Game Making

you can also try to make your own combine mesh and material script

Or you can use the default methods to optimize performance, Some Usefull Links: How to opitimize Shadows in Unity 3D - Game Development Stack Exchange OR
What are causes of "Overhead" in Profiler? - Questions & Answers - Unity Discussions OR
General Ways to Optimize Frame Rate? - Questions & Answers - Unity Discussions

A few tips:

Use textures to “fake” detail. From the images you provided, it’s clear that you’re going to paint in the windows and stuff. Instead of having a texture for a wall, and another texture for a window which is going to be painted over the wall, have 1 texture for the wall with the windows already on.

All vertices will count. Even cutting down on a few vertices on each model will improve performance dramatically. Also, keep it as low poly as possible (eg. avoid round edges).

Also, when it comes to colliders, I’m against the use of mesh colliders (unless they’re convex). use box colliders for the general outline of the building and then smaller box colliders for the most important details.

I hope that what I said makes sense.