Give it a try… we had the same issue, basically a huge town with lots of actors/buildings. You can make the levels as small as needed, e.g. we have a Dam with about 2000 actors inside a building. So the outside walls get loaded once the player is in the area, the interior gets loaded when the player gets very close to the building. Also, there is a “merge actors” function in the editor on experimental. If you use static lighting you can also use a precomputed visibility volume. But splitting up your city into sections should already help a lot.
You can also use simplygon to create LoDs and merge materials into atlases instead.