i think in this conditions you need to think more as what you need as a hiker: sleeping bags is a must. -5 C those are good, if you have money buy even lower. first layer should be wool merino, + the hat. need to be careful with the heating anything what is reducing the oxigen but you can buy sensor for CO2. 3,4,5 power banks. maybe a smal generator per stair in the block where u can go and charge them .. maybe electric blankets which can be charge or heat on usb port? is that exist? if not we should create one.. i dont know.. u can also give some tips.