Hi Ramon, in my opinion, and comparing your boat and mine, I dont think you have too much of a ballast issue. THe two boats sit pretty much the same in the water, this hull will always do that due to the shape. I think the big problem with stability is the placement of the rudder, it could be too far back, as the trailing edge is past the transom, so when turning, it is almost using the transom as the axis, rather than further up the hull, if you know what I mean!
If you have a couple of cheap rudders, I would try narrowing the blade quite a bit, and see what diffence it makes, if no good the rudders are only cheap. Extra ballast seems a shame, as you are adding weight, and slowing the boat down, if you run at a slower speed, and it still handles/turns bad, then that eliminates ballast. Sadly, you might have to move the prop back a bit, and the rudder. Here is a copy of mine pre final paint and set up. its copied from the original plans, but the angle has now been changed, but it gives you an idea where it is in relation to yours. Dont yet have a good photo of it sitting in the water unfortunately, but it is almost identical to yours, at 6kgs all up weight