g_a_w:
The GRA you quoted says the Tznius is the main anti-yetzer hara potion for a woman, not that it is the “ikkur schar”
You are right. I am being influenced by a leaflet that my wife was given 4 years ago about tzniyus. The ikkur schar a women gets is from sending her husband to beis medrash and sons to cheider.