RE: advice reg. buying my 1. Starboard
I'm surprized, since you are sailing on a Sonic (what size is it..?) that you want a board with a centerboard.
But, for 3-6 m/h (is that miles per hour...?) the Hybvrid Carve does nicely in displacement (longboard) mode with it's fairly large and efficient centerboard.
When the wind gets to about 12 knots, with a 6.9 m2 rig you should be able to retract the centerboard and go planing.
Yes the board is fairly big (about the same width as the Carve 161,
or a smaller GO board), and kind of heavy (due to the big centerboard and the casette for the centerboard, but it does so much better when there's not enough wind to plane.
It planes pretty early, and is reasonably fast when on the plane, but nothing like the blinding speed of the Sonic you are sailing.
If you "MUST" have the centerboard, and can't wait for spring, then the Hybrid Carve (if one is available in Norway) would be a good choice for you.
Try to demo one if you can as I'm not sure how you are going to like the weight, and the difference in feel vice the Sonic.
Hope this helps