The search term you are looking for is "Level of Detail" or "LOD". It basically means that you adjust the level of detail of the map depending on the zoom factor.
You can have a really high level of detail on the highest zoom level. When the player zoomed in so far that there aren't many objects on the screen, you can get away with that without losing too much framerate.
When the player zooms out they can't discern that level of detail anyway, so you can remove small objects and replace larger objects with lower-polygon versions. That way the framerate stays acceptable and when you do it well the player won't even notice.
The Clausewitz engine by Paradox (the graphic engine used by CK2 and EU4) in fact does that as well. Compare how a city is rendered on these three screenshots which were made from marginally different zoom levels. As you can see the buildings are first replaced with simpler models and then all but the largest one are removed altogether:
The Unity terrain system already has that built in (including for the speedtree-based trees). For other 3d objects, you can use the LOD group component.