I bought some Castelli gear just for my SS. I use their long bibs ( wind stopper and thermal ) merino socks and either their neoprene overshoes or their wind stopper ones, then I use a merino long sleeve T, a LS roadie riding jersey, then either a wind stopper gillet or a Rapha wind stopper rain jacket, then gloves are either neoprene or wind stopper.
The only part of my body that has gotten cold on long SS rides are the tips of my toes and tips of my fingers.
If its proper wet and muddy I use a lot of old kit and just layer up.
It’s similar kit on the roadie but consists of Raphas deep winter tights, merino T, deep winter wind stopper LS jersey or rain jacket, gloves I use my Castellis as above.
One thing that works for me though is embrocation. Rubbing this on pre rides in this horrid cold weather has made my rides soooo much more fun.
Oh, for hats I either use a winter merino beanie or a casquette under my helmet. Or some other thermal beanie I have lying around.
And shades, well clear lens shades, just to keep the wind off my eyeballs.