I surmise limitation of window motor is not only too little power [torque & amps] but having it's own gear reduction, RPM too slow for converting up into usable feed rate? The fan motor has all kinds of RPM so your gearbox puts out super reduction needed for milling, not just moving X axis putting here to there.
I'm not surprised, recalling that 18 speed gearbox you hung on the lathe. You have power transmission down-pat!
All along I thought your shop was onboard a vessel...until I saw hinged windows and plaster.
Too funny.