A one board solution for me for 5-20 kn would be a GO.
If you make that 10-22 knots i can agree. Because in less thezn 10 knots the Go boards just float along unless you start using enorous rigs which are not fun to drag out of the water and make sailing the Go a bit too technical for the ones that use one. I'd like to buy my Go139 an 8.5 rig so i can start planning in 7 knots or so, but once i have to start thinking of 9 m² rigs or above, i think you might just aswell take ur 8.5 or even 7.5 rig and start pumping your longboard cuz it'll be a lot faster than any shortboard up to 13 knots because of it's glide.
I do have to agree with LK on the statement that the longboard is the solution for most surfers as it is more enjoyable for much longer than other boards: if you compare the windrange of a slalom board, a formula board or a freeride board you see that when the wind picks up you can keep using them for much longer. the downside: you need the wind to be blowing over about 12 knots before you can take them out and blast. But once on, you can keep sailing a shortboard up to 25 knots or so. A slight difference is found in freestyle boards and wave boards but we'll leave those for the ones that frequent those conditions.