[ianlevesque]

Better insulation was missing from your list (or was maybe part of "etc") but if you want a more efficient heating / cooling system ground source heat pumps are an option. They can run from the grid more efficiently than most other sources of heating or cooling or optionally be combined with solar.

[bcarrell]

Walls were stripped to studs approx. 8 years ago and new (fiberglass) insulation installed at that point. Attic has batt insulation.