Unregistered, I think the easiest way to get the most bang for the buck is to buy something that increases your time on water (TOW). Lengthening your possible season has the best shot at increasing TOW. If your water or air gets cold enough, the most effective items to extend your season are (in effective order): a hood, wetsuit or drysuit, booties, gloves or mittens. How many times last year did you have sailable water, time and opportunity but didn't go because you didn't have good enough protection against the cold? Consider spending enough on better protection to convert about half of those opportunities to TOW.
I particularly mentioned a neoprene solution because where I live 8-15 mph is what we hope for in the summer. Our best chances for 15-25 are in the spring and fall when TOW is directly related to the quality of what we wear as protection.
Someplace in the budget or toolkit there must be a downhaul tool. You need it to properly rig the much larger sail(s) you will buy.
Someplace in the 8-15 mph range there is a wind speed where, with your largest new sail and your widest new board, you will start planing. If the wind you sail in is most often less than that wind speed, you might be most satisfied buying a large volume longboard, new or used. TOW is one issue. Planing versus cruising versus schlogging is an equally important issue. With your weight and a 180 ltr longboard for comparison I think you would notice a significant improvement in cruising speed and also in planing time with a longboard volume at or above 220 (you want more than 220). The dream boards in that case would be a Phantom 380, Phantom 320, or a Mistral Equipe.