Greatcoats actually work well on a bike when it starts to get pretty cold (ca. -20°C), I have a thick wool one that is Russian navy surplus and I might look like I just biked off of a warship, but it keeps me warm yet also breathes if I get too warm. Also, it’s long but splits down the middle nicely when I straddle my bike.

I haven’t biked in much colder than that, though in the winter it easily gets to -30 to -40 here. I kinda want to try but it’s quite intimidating both in terms of keeping all my extremeties as well as all the ice.