Another former Alaskan here—-7 winters in Fairbanks, where the temperature could reach 60 below, but mostly was around 20-30 below. The only boots that kept my feet warm at those temperatures when I was outside for long periods were my “bunny boots”, which I finally found in a small women’s size at an army surplus store.
https://bootspy.com/bunny-boots/
I am certainly not recommending you get a pair of these, as I doubt you will encounter those extreme temperatures. But one can learn from these boots, such as how important it is to have sufficient insulation underfoot—like a thick sole and a felt liner insole. I like my foot to be at least an inch off the cold ground, and that means a think and possibly stiff sole.
For warmer temps, around zero F, I like Sorels. They are what I continue to buy for winter wear in snow country. My current Sorels are not the traditional rubber bottom, leather top style, but all leather with Sherpa fleece lining. Not as warm as the ones with a removable felt liner, but fine down to zero degrees. And no rubber at all, apart from the soles.
Note that if it is really cold, as you imply with the words “ultra cold”, waterproof is not essential, as water does not exist at those temperatures . It is all frozen (ice or snow). If you brush off the snow before going indoors where the ice will melt, your boots will stay dry. If you look at winter boots worn by traditional cultures in the northern Scandinavian regions, you will see wool and felt, maybe part leather, but no rubber, apart from maybe on the soles. So an all-rubber boot may not be what you want.
Thick wool socks are part of the equation.
