One of the Go boards, or one of the larger Carves/Futura's would be my recommendation.
Are you sure you are truly "tuning" your Rio L as the wind and chop come up so that you get the smoothest ride in higher wind conditions.
At your weight, in 20 knots of wind, the 6.5 might be a little big, but if you are hooked in, and in the footstraps, I would think that with a little mast foot tuning (move it back to nearly the back of the slot) and a larger fin, you would be getting a pretty nice ride on the Rio.
But, you wanted to know what board to progress to.....
Probably a Carve/Futura in the 140 liter range.
If you still want to go for the early planing with your 10.0 m2 Retro, you may want to consider a larger GO, but that's going to be pretty close to the same as your Rio.
Hope this helps,