Hello, we also surf in Finland during the extremely cold winter. Mittens are the most challenging thing. Personally, I use 5mm mittens in cold weather. In my opinion, the most important thing is that the material on the palm side is very grippy, such as rubber or similar.At the moment, it's really difficult to find mittens with a grippy palm. Sales pictures and promises often claim that the palm side has good grip, but even in expensive brands, it tends to be very slippery. That's why I've personally improved the grip by adding neoprene glue to the palm side. It creates a rubber-like surface. With mittens like these, I can surf for several hours, just like in summer. Of course, it would be most comfortable without mittens."
It's advisable to start using gloves well in advance and gradually so that the wrists have time to adjust. However, my wrists are completely normal. I can only windsurf for a couple of minutes with fully enclosed gloves, but I can engage in wing surfing for hours.
Here is more info about my winter setup:
wingsurf.fi/en/wingsurf-wingfoiling-accessories-wetsuit-harness-leash-pump/