Socks, thick wool socks in keen shoes + neoprene overshoes are doing the job for me. I add the overshoes around -15C or when it's very windy.

My bar mitts experiment on flat bars is still going strong. So far, I have barely had cold hands when they were warm going into the ride. And that is with a single layer of gloves. For the next few days (forcasts are -22C with feels like below -30C), I'm expecting that I may have to add mitten to stay comfy warm. For me, this is a completely different world than using 2 layers of wool/fleeze gloves/mittens. It's not about being numb or just cold, it's about comfy warm or cold. Downside though: sweat management. I have had sweaty gloves a few times which means frequent glove washing and being careful outside after a ride, if I stay out.