Obviously an air exchanger is needed. I needed on in my lake home because it was masonry and use too tight.
Windows are just a hole in a wall and all the films and gasses between the glass layers do little and possibly get the insulation value up to that provided by a plain 8" block with no insulation. The aluminum frames are dramatic conductor that will always be worse than any other material. Frost is a visual clue to the the localized heat loss coupled with excess moisture and poor circulation. - Even in the cold north here, frost on a window is rare in a newer homes.
One basic problem is that the steel frame is virtual radiator and conducts heat/cold very, very efficiently. The fact that is welded to the beams magnifies the heat loss and the energy demands on the heat pump and the associated needs for circulation. It is good you did not use steel studs. Your R-19 insulation is really about R-17 (for wood framing) or R13 (for wrinkled tin studs).
Get a professional (not a HVAC man or insulation contractor) to look at your creation and ask for suggestions.