ST CG is right under the main spar, 5 to 5-1/8" behind leading edge. I presume your lg wire is 5/32", not 5/16". My ST came in 9 lbs 15 oz, so 10 lbs sounds good to me.

I can't imagine why you have too heavy a tail--there just isn't an awful lot of wood there, it seems to me. I added the truss sticks, as mentioned above, and still had no problem. In fact I gave it a Robart tailwheel, a bit heavier than the kit-supplied item.

Maybe you ended up with heavy wood in the vertical tailplane. If you can't get close to CG otherwise, then consider a stick-built tailfin. I don't imagine you are going to be knife-edging this plane, or otherwise overstressing the rudder.

I should mention that if you need roll control at low landing speed, rudder will give you better control than aileron at low airspeed (and better tip-stall resistance, if you have that problem).