A formula board would increase your early planning, but you won't get enough at 5-10 knots. This is the realm of longboards. The top of the line solution is probably a Serenity or a Phantom 380. But almost any longboard of almost any vintage will give you more overall velocity made good than a formula board across all of the 5-10 knot wind range. An Apollo plus much larger rigs might make you happy around 10 knots or maybe less. Only the glide of a longboard has a chance of making you happy at 5 knots and keeping you happy up to and maybe beyond 10 knots, especially with the fluctuations in wind strength found on inland lakes.
I sail in quite a few races in the Midwest in the Open class. For races at 5-10 knots out of a field of about 30 racers there will be 1-3 on an RSX (maybe none), about 5-10 on a Prodigy or similar (maybe only 3-5) and all of the rest will be on boards like Mistral Equipes or One Designs including 1-3 on up to date boards like Starboard's Phantom 380.....notice no pure formula boards.