How can I determine the amount of trim drag?
As I mentioned in my comments. You could test this empirically with the experiment described there.  If you find the additional ballast improved the glide performance of the craft, is there an aftermarket STC kit available which installs new ballast tanks in a more optimum position for a more neutral cruise trim?

I am a little concerned with the idea of continuing to move the CG aft of its current position in order to optimize the performance at the expense of. Asking the craft less stable about the lateral axis.  No increase in performance is worth your life.