Before I buy I want to make sure I'm not going to have to tediously remove vertices & faces 1-by-1 in blender after making a model to simplify meshes.

For instance, if you make a larger cube out of 8 cubes, it would be expected that the large cube would still only be made out of 12 tris and 8 verts, with no internal geometry. Instead of now having a cube with 96 tris & 64 verts.

Does asset forge do this?

No, Asset Forge does not do this automatically for you. We've experimented with this in the past but did not get consistent results as it is nearly impossible to automate this process because of edge cases and the chance that it might create more vertices than before optimization.

Asset Forge can merge blocks so they're a single mesh, however it will not remove internal vertices.