I tend to wear Ridgeline gear, designed for outdoor people to sit in all weather and enjoy the outdoors. They work on the layer principle. During winter, I wear a thermal layer, then a fleece, then a thicker fleece with a windstopper and so on. On my feet, I wear Oakley boots (I fell in love with them at work), thin socks, then a thick thermal sock inside my boots. Don't get cold feet flying the plane or standing in place when flying the drones.
You need to remember to move your feet and legs as well. If you just sit there and don't move them to keep the blood flowing, you will get cold, and it will be hard to warm up within the confines of a small plane cockpit. I am a fidgety person anyway, so I am always flexing my feet, etc.
Another big one for retaining heat is your neck (especially the back) and your head. I have either a scarf and/or neck gaiter for winter and a thin thermal hat that fits under my helmet (and I block the air vents with tape in the helmet).
As you get cold, your body will reduce the blood flow to your extremities to retain core heat, so staying warm and not letting your core cool is critical. Vests, layers, etc.
As a kid, my mother felt the cold, and she had a small metho-powered pocket heater that was the size of a hip flask. You started the thing up, closed up the box and put it in a velvet bag and put it in your pocket to help keep core warmth. Not sure if you can still get them.
Greg