i always used to get pulled pretty much a full line legth when launching downwind on my 12 mast air, until i started doing this.
instead of digging your heels into the sand to stop yourself being dragged. do the opposite. digging your heels in loads up the lines and increases drag.
get the lines tight, hook into the depower, go on its safe, and step back and depower as soon as the kite rises walk towards the kite, this lowers the line tension, lowers the pull and slows down the kites rate of ascent. you'll walk as far, but it will all be under control.
think of it another way if you let someone hold onto you and then release you when the kite is half up you'll take off like a rocket. this is basicaly what you are doing by digging in.
hooking in might seem scary but it is safer. if you only have the old none release chicken loop, email armin and tell him that you are stuggling launching the kite and he'll mail you a new chicken loop out, i'm sure, flysurfer really do care about their riders, ask anyone. try your dealer, they migth have replaced chicken loops of the warrior depowers...
i would also consider moving the knot down a couple of cm to the bar, extra depower is good for launching and actually gives you a bit more upwind ability as well, as you can oversheet the mastairs quite a bit, safely. just remember not to use this full depower directly overhead as they can luff set up like this without the drag of aparent wind generated from moving sailing.
this is really a matter of arm strength, but there is technique to help.
form the unhooked position, hands on either side of the bar, chicken loop pulled by the kite into the bar.
slide both hands into the middle of the bar. slide one hand so it straddles the depower in the middle of the bar, sink to your knees. take one hand off the bar and grab the chicken loop and then as you stand up pull both hands down and hook in.
holding the middle of the bar like this means you are pulling the kite down as well as the depower, reducing the pull on the depower as you stand up, the difference between kneeling and standing up should give you the movement and slack you need to hook in.
but defo' get a qr chicken loop, if you feel safer you'll be prepared to try harder.