Advice on supporting a sagging bay window
I cringe when I see this type of construction. How good are you at mixing concrete? I'd be digging out that area under the bay window, compacting it, adding a few reinforcing rods into the  existing foundation, a little framing and pour a 6" slab/footer. Then use your jacks to slowly raise or even over raise the window, cinderblock and morter it in and cover with a coat of stucco. Don't forget to extend the vent with a prefab from your home store.