I did not realise you have brambles and I would dig these out with a sharp spade, just the root crowns and then have a bonfire. That should make a huge difference to clearing. Docks I have also used a sharp spade for to remove the top 4-6" and I find they do not re-grow, though others have.
Then a light excluding mulch. I would not use the Roundup and as others have said here, there is more and more evidence emerging of its pernicious effects.
Good luck with the rocks!
