And the Founding Fathers made it difficult to have many parties at the Federal level (which is fine).

The Founding Fathers made sure that no government would deny or disparage rights held by the People. Long story short, the State cannot declare that Proportional Voting is verboten at the local level.

The State itself should actually have Proportional Voting for State elections, but the Founding Fathers gave them freedom (read: loopholes) so the demand cannot be made, but in the future we can change it quite easily ourselves if we all desire to do that per each State.

It is okay to have a 2D voting system at the Federal level.

It is not okay to have a 2D voting system at the lower levels of government. We are entitled to a 3D voting system at the local level, and we can get it at the State level later on.