Inland lake conditions of 8-10 knots 90% of the time, just shouts to me that one of the answers is: buy a longboard. (Those are the conditions I've sailed in most of the time since 1984.)
At non-planing speeds the physics of waterline length mean the GO will always be shloging while a 12 ft longboard will always be gliding. Since you already know how to sail a windsurfer switching from the GO to a more tippy longboard will be a short transition.
Adding an 8.5 or larger sail probably means going to a larger mast and boom as well. Buy the longboard first because it will add more pleasure to more days than a larger rig. If you can't find an old longboard cheaply then consider a 12 ft SUP.