shouldn’t you be able to go to 240?
Not really. The faces of the D120 are already asymmetric (scalene) triangles. There’s no way to split these into additional faces while keeping them all the same shape.
Any larger geometrically fair die would have to be from an infinite family (prisms, bipyramids, and trapezohedra), which are “impractical in reality due to the tendency to roll for a long time”.
Yes, but since faces are essentially distributed over a cylinder, rather than a sphere, it is more of an issue.