Usually to render a large quantity of gird aligned cubes is to use a method known as "Marching cubes".
Note: the main alternative is to use ray casting and voxels (point cloud data), which is less common in video games, however it is the standard in medical imaging. See also: A Fast Voxel Traversal Algorithm for Ray Tracing.
You will see "Marching cubes" ...
You don't need that extra geometry to apply a bevel effect via normal map. Just bake the normals of a high-res beveled cube to a simple cube in Blender (free software). There are many tutorials for this on Youtube.
I don't think it will look incredibly realistic but it will not look worse than what you have with more geometry, and require only 6 faces.