[spaghetti]

Adding another problem to the ones mentioned by Dave, I guess a 30 kilograms ballast would pull you down so fast that you will scare the fish. (And the ballast itself is likely to make noise when touching the bottom)
One of the golden rules of spearfishing is: act like a fish. And fish don't swim fast, unless they're escaping or attacking: in any case, a "creature" moving fast is a danger signal for them, a signal that something to be wary about is going on.
However, if you feel the need to use some releasing weight technique, you could simply go classic.
A weight belt with, to say, some 8 kilograms (depending to you body weight and suit thickness) will pull you down pretty fast after just a few finstrokes in the first 2-3 meters. If the belt is tied or clipped to your float line, you can release it before ascending (gaining a shorter ascent time) and easlily retrieve it by pulling the rope.
This is the classic, popular and widely field-tested technique for hunting deep. Down with belt, release, ascend.
Even inside your 10/12 meters SAFE range it will help a bit.