Soul 10m is not build as light wind kite but as a high wind boosting kite with structural endurance. True that with foiling we misuse (many) kite's original design intention, Soul = TT boosting durable kite with easy step-in from LEI side.
I always talk here @KF about the force impulse
to get riding, which is "force x time-the-force-applies
", meaning a) can you get on top of your board from water b) can you accelerate your board to hydrofoil take-off speed and finally c) ride away to sunset. So in light wind which kite provides the required impulse (force x time of force applies) for you to ride. From Peter_Frank's statement "Doesn't work, no peak power, have tried both." my own educated option even gets stronger. I possible I'd go even for Flysurfers VMG11 kite to be more efficient like sail planes are.
Many double skin closed cell foil kites do work under 10kn, but there might be differences in way you get the mandatory impulse
to get going. Sonic3 15m, easy dive of kite and you ride in ultra-light-winds. Peak4 you might need a lot of looping and very delicate bar position control to get out of water and accelerate board&foil to flying speed in similar situation, like Horst Sergio's video on youtube displays like 5 kite loops or so (from faint memory, don't count from the actual video
How do I know, well Regis, I am almost the double of your weight so my riding problem and required impulse levels to ride are double compared to yours (linear mass dependency). So we can directly calculate force difference on potential energy change to get out of water to top of board (my mass is almost 2x of yours and my center of mass is probably ten(s) of centimeters higher than yours for potential energy change), then kinetic energy change to accelerate to planing and hydrofoil flying. Also on other thread here we have talked with Peter closed-cell-foil kinetic energy vs Peak4 kinetic energy which is a sort of flywheel effect via inertia to create force impulse required to lift off from water to ride with foil. I interpret that Peter finally agrees me (ref. previous Peter's quote "Doesn't work...").
I wonder your drive to 18m kites (with lower aspect) if you are 60-70kg, when I with almost double of your weight and required force impulse to ride I can do my stuff with 15m foil kites (already a couple of generations ago model) in proven light wind **with door-twintip when "LW"-LEI kites cannot climb to zenith**. IMO you don't need 18m kite's impulse to do your HF stuff, actually smaller kite with less fabric weight & air-inside-mass-inertia and easier to achieve higher flying speed and quicker steering with easier-to-fly-arcs would be much better for you maybe with 23-25m lines to create window room kite to accelerate and generate the impulse. My opinion is that 18m is a waste of fabric for "60kg" person, it was already in pre-historic-dinosaur-twintip-era when our local 60kg-kiter with 19m Speed3DLX was saying it is too much force with twintip already for their weight in our famous light winds. I am 110kg and 18m kite is waste of fabric for me.
I'd prefer using more efficient high aspect airfoil with less inertia to go if I were you for light wind. In my case at your weight I'd probably have Sonic3-11m (recreational riding with std. 21m lines) as my max size kite. Now my many generations old Speed4 10lotus kite is most usual HF-kite I go and only when I cannot get impulse required for initial HF flying I swap to Sonic3 15m which still delivers. Peak4 5m is about the similar required wind speed for me to HF-ride compared to Speed4 10lotus on initial impulse level but never did direct 1:1 comparison swapping kites.