I just ware a thin fleese sweatshirt under the snowboardjacket. No thermo underware, just the snowboard trousers. But helmet and face mask are standard even in spring. If it's below 15°celsius, I might add a second shirt. But mostly I'm sweaty anyway after the first descent no matter how cold or warm it is.
I always take along a spare fleese west in the backbag for the less active aprés ski