I just started experimenting with the really light wind stuff myself, got 7-10 knots sussed no prob.
Now I'm aiming for less, but you're already doing better than I am. I do weigh 15-20 kgs more than you though and am definately up for 6-7 if not 5-6, I love challenges.
I'm also using 30m lines on the 12m but will soon try 10m extensions, Flysurfer say you should get 1-1.5 knots more low end.
Strange you say it's a slow kite, mine is charging around, in low wind you do really have to fly it around though to give it apparent wind, this also helps stop it falling in a lull, although if the lull lasts long enough no kite will stay in the sky.
Things should be perfect, light constant wind, flat water, flawless kite and board technique.
If it's gusting with big blody holes forget it.
Mind you the other day I saw a guy buggying with a C-quad I think, wacking the bugger all over the shop in NO wind, really 0 knots and the kite generated enough pull to get him moving. Obviously as soon as he stopped sining it just glided down, it was a good lesson seeing that.
Open cell foils fly better in light wind and anyway bellow 7 knots you can't water relaunch anything, maybe you should try a big open cell foil.
I agree that the 12m is a bitch to launch in less than 7 and I'm experimenting with various methods to get it up, without dragging it across the beach, can't be good for the trailing edge. Running backwards with someone holding the kite does actually work, fully depowered. Pre-inflation is really important too, I use the velcro butt plug, kite fills almost full on its own while laying out the lines, amazing stuff.
Try riding a surf longboard, about a 9 footer
Get it up in light wind.