I ride until about -12C (10F).
Multiple layers but an absolute must is a wind resistant outer layer.
At the lowest temps, I swap out my normal Fox helmet with beanie underneath + wrap sunglasses, for a K2 ski/snowboarding helmet and goggles.
Yes - a balaclava is often in the mix and so are proper ski gloves.
I will find out this winter how a Garmin 530 handles the cold. I've frozen my Samsung GS7 multiple times when I was using it on a Quadlock.
New GS22 stays in the pocket now...
Thankfully we're having a beautiful Autumn, and I haven't had to organize my winter duds yet...
But when it arrives, and it will, thankfully our city has a pretty good bike path cleaning program, so I get to stay off the streets.
Note - I have a complete second set of wheels with Ice Spiker Pros ready to go, so I can change the running gear quickly for changing conditions.
Living this close to the Canadian Rockies, we can get some pretty wild weather shifts.
I've ridden in +10C in the middle of winter - you still need to layer up as the ground is frozen by then and the wind is cold no matter what.