[Renze]

A force of 1 ripper leg on the side (instead of all 6 or 10 of them spread over the full width) wont hurt the tractor nor the 3pt hitch.

Can you extend one 3pt hitch pull rod to angle the box blade ?
another idea is a kind of plough i've seen road crews use, to plough a pathway along freshly paved asphalt, for kerbstones.
They simply bolt a 2 foot piece of steel plate to a loader bucket and drive along the road edge, with the bucket lip as a gauge over the pavement and the steel plate ploughing a path for the kerbstones at the correct depth.

Maybe you'll just have to tramp the brush first in reverse, then start working the actual dirt...