Shen: Our cars ARE double a-arm/wishbone, STB's are appliciable to McPherson Strut suspension where the towers are load bearing in cornering, ourws aren't all they do is support the spring loads nothing to do with cornering.
If you look at most STB's they're designed to be easily unbolted and this is their downfall as a brace as they act as a big ass hinge at the single bolting point.
Think of a cardboard box, unfold the ends so it's a tube then grip diagonal corners and flex it side to side, this is all your STB could possibly do as installed.
Fold the ends back over and repeat, it's pretty strong this way and this is what an STB is missing to be effective as the engine is in the way.
Run a couple of diagonals back to the firewall and continue them into the dash support brace and it might just do something.