Cold fingers is a real problem. We aren't as cold out here on west coast, but we do get cold snaps. I find you can keep core warm pretty easy, especially if you add an impact vest or something to cut wind chill. But fingers can be a session limiter / ender. For surfing we wear 5mm gloves, and I know easterners wear the mittens, but that leads to forearm fatigue. So you have frozen fingers or can't hold on, either way it doesn't work. Below 3-4c is now pretty much my limit.
Some options:
-- use of harness helps massively. You can lean back against it and get your fingers moving more, shake you arm to get blood flowing.
-- cold military training: they use a technique where they expose fingers to cold for 5-10 mins, get numb, then a quick warm up 5mins, after that you're good for 1-1.5 hours. But that warm-up costs you brief intense pain ("hammer finger" I call it, like getting smacked with a hammer). I have been told it's quite bad for your joints too, arthritis, etc.
-- webbing: my Ocean Rodeo wing has nice cushy handles, but they create arm fatigue with gloves. My old bagged out ozone straps were pretty much webbing and I didn't get fatigue, as you don't hold on to them, you kind of hang off them. I have thought about adding webbing below straps as a way to rest fingers/grip.
-- tubing: back in kiting days I played around with idea of putting in tubing from neck to gloves, to periodically blow on your fingers. Never bothered to follow it up and properly test it, but I think it could work.