I predict a bit of flak for this, but here goes nothing..........
I don't accept that recreational cycling is pastime that should be undertaken in the extreme cold. By that I mean under 3degC. If there is the slightest risk of ice on the road, I don't bother.
However after many decades of riding, certain truths become self evident.
Body shape more than body mass is a major factor in keeping warm. Lanky types get cold, the more rotund riders do not. More to the point the skinny types do not have the skin area/body mass ratio to retain heat.
I don't see the benefit of riding hard in cold weather. The taking in of massive amounts of cold air is not a positive activity, also generating sweat without dispersing it effectively lowers the rider's temperature.
The hands, feet and other priority areas are difficult to protect from the cold. Your hands must be able to work the levers and feet must be comfortable when stuck to a pedal. Wind rush produces wind chill.
The base layer is the big priority. I use
NIKE dri-fit and have found it most effective, bit I'm sure there is other stuff around just as good. The garment I use this weather is a long sleeve version of my summer ones.
Bugger using a a helmet - an effective hat that covers the ears is my number one, especially with a buff pulled up high at the back. Gloves are now very good but the wrist and fore-arms need to be protected as this is where the hot blood comes from in order to get to those otherwise frozen pinkies. Overshoes are no good unless effective socks are used. I have several pair of Damart long sox which protect the calves. These with Adidas neoprene overshoe mean my feet are never cold.
Also, I never stop (voluntarily), you never seem to get going again.