Hi, If you keep them as instances - but use a singleton instance manager so all instances in all buildings use the same ISMCs, you can get slightly faster render times compared to rendering a complete mesh (things like culling, distance from camera etc are taken from the instances rather than the complete mesh (or with nanite, different culling techniques))- and keep the benefits of smaller modular parts (less VRAM, faster to build/change etc).
Collision built from modular parts takes away the pain of building collision for internals etc too.
If you’re adding destruction to the buildings - that may be something to consider too - how you’re going to build the geometry collections etc.