It will work if you are going to plant shallow rooted plants such as succulents.* Since you are posting this in the veggie forum, it won't work.* You can't successfully grow veggies with roots of a thorny plant under them and weed cloth over that.* Then you'll have 6" of added loam to plant in?* How can you put weed cloth over the soil and then mix it with the added soil?* Besides, if you till all those roots, you'll have thousands of plants if you mix that soil with what you bring in.* You'll just be chopping them up so they can sprout.* I'd say to remove as much of the roots as you possibly can, add 4" of compost and mix that in.* I don't see why you need more soil as you already have that.* Just enrich it with compost.
Have you had a soil test done?* What type of soil do you have - sandy, clay, loam, etc.* Any idea what the wild thorn were - raspberries or maybe roses?
Maybe this will be helpful.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.