Unregistered, your weight is 1-2 kg lighter than mine. No one board/one sail combination that you would love at 15-20 knots will make you happy at 6-10 knots unless you are a dedicated one-design racer. I'm having trouble picturing anyone having fun trying to going against a current on a small board with a small sail in winds of only 6-10 knots. The board/sail combinations already mentioned could be great at 15 to 20 knots but they won't entice you to get on the water in 6-10 knots.
I think the only claims I've seen for a one board/one sail combination that can handle 6-20 knt condititions at our weights are: 1. a formula board with around an 8.5 to 10.0 sail, 2. a hybrid such as an RS:X or a Mistral Prodigy with whatever their class size sail is for the particular board, 3. an older longboard such as a Mistral One-Design or Mistral Equipe (I or II) with something like Mistral's OD 7.4 sail, 4. a Kona with their one-design class sail (7.5 or 9.5?), 5. a newstyle raceboard such as a Starboard Phantom 320 or 380 with the one sail or your choice (8.5?).
My choice would be a board large enough you won't just schlog at 6-10 knots and then have two sails, one for 6-10 and one for 15-20.