Weed/Grass Spray for Gravel Yards and Roads?

Seldom driven parking areas get grass and weeds and blackberries that start to grow in spring. Summer usually cooks them dead but next spring they come back more and more. And start to grow through my treasures. About 1/2 acre is what I need to treat.

I would like to apply something like the county uses on their road shoulders. I have a backpack sprayer. What should I be looking for? Roundup or some other brand? And how to get it economically?

Weed/Grass Spray for Gravel Yards and Roads?