You made one single mesh that is either displayed or not, not much optimization can be done here.
Engine already has instanced mesh actor for such tasks, and its optimized.
Now it all depends on your tiles of hex grid, if they really procedurally generated (ie. no 2 tiles are same) or you use tiles like civ games have (ie. same hill tile just rotated and painted).
In first case you cannot optimize it much more, besides lower polycount, some world aligned material (not sure if this gives you any optimization), probably splitting that map into meshes can give you bit more speed on rendering but will waste as much if not more on loading data to gpu.
If your game will use true tillable tiles, then there is map generator thread, you probably get all possible tips there about instanced meshes and creating civ map. It is done with blueprints but you can transfer all ideas over to C++.