In the northwest corner of Massachusetts I converted an old school into an apartment building. I installed 2" of polystyrene on the outside and about a foot of cellulose in the ceilings. We relay on heatpumps for HVAC. I also installed a 50kW solar array. We don't start paying for heating until Nov/Dec and stop paying in Apr/May. Our Electric usage goes through the roof in Jan/Feb/Mar. Our weak point is that the exterior walls are about 40% windows. I hope to install better thermal shades which will cost about $80k. We also last fall installed a solar thermal array to for hot water and heat the hallway which is radiant floor. I would like to think we could achieve net-zero but I will likely need to expand the solar array by about 200%.

Thermal curtains are more effective than good windows. Good windows are minimally helpful.

Thermal curtains are a godsend. I remember reading about your journey and I hope it works out! I think it'd be money well spent.

In my last house, I replaced single pane windows with properly installed, sealed, and insulated double-hungs and it practically cut my heat bill in half. I agree that modern window to modern window replacement probably won't get you much, though.