For a light weight, key for planing in lowest wind is big sail and highly active pumping skills. Considering your intermediate/advanced skills, the Apollo would be far more easier to get planing in low wind because you'll need far less active pumping than that with a pure Formula like F162/161/160. Check out old forum posts by Krister and his experience between Apollo and Formula.
The best combo to plane at or slightly below 6 knots true wind is Apollo + 75 cm fin and a big freerace and soft sail like Overdrive 11.0 IMHO. I doubt someone has ever achieved planing as of 5 knots true wind, any reports ?
If you want to go sailing in any wind lower than what you can achive by planing with your Go and 9.7 sail, a Serenity is a nice complement to your current Go but in non-planing conditions (and a lot funnier, faster and far easier than slogging on a Formula, believe me). However, I'm unsure whether your no cam 9.7 sail would be a good match or not. Problem is you'll need a fairly baggy shape to be efficient in very light wind on the Serenity. Cambers do provide such a baggy shape even in zero wind. Without a camber, you'll need to wait for the wind to get a belly shape in your sail. In addition, very large camberless sails are much less stable in the upper wind range than their cambered counterparts. I'll leave that to raceboard specialists to further comment (modern raceboard sails do have cambers).
In sum, how many times is it blowing 2-7 or 6-12 knots at your place in the summer ? If 2-7 > 6-12, get a Serenity with your 9.7 sail. If 2-7 < 6-12, get an Apollo + 11 m2 sail.