If you can purchase a disc plow in small enough width that is powered
by the rear power take off that would be faster than a tiller.
BUT saying in saying that the issue is any grass left un-covered adding to
the fuel load.
A larger tractor and PTO powered disc plow would be even better as the cut would be even deeper.
Another possibility to consider is a rotary plow.
In one example:
A 13 horse BCS two wheel tractor with a rotary plow will till 14 inches deep in one pass with the wheels
set at their maximum width at the first pass and every pass thereafter
The dual rotary plow will allow you to turn back on the same furrow and go back rather than turn around and
return to the starting point to till the furrow again.
A single moldboard fitted with a Weisse conservation tillage plow share will also plow down 14 inches
with good suck and the big plus is very low drag and it will do this with very lttle effort due to the plows
high trash clearance and very low drag.
Just a few options to illustrate the possible options to you as a pto powered disc plow may cost more than
a new 13 horse BCS two wheel tractor with a dual rotary plow.
EDIT: My aplogies for forgeting this:
1.A four wheel drive Ferrari Orchard And Vineyard Tractor with a front mounted flailmower and a rear mounted tiller would also work well
as it is a very low profile rubber tired utlity tractor that has an articulating steering sytem(swivels in the middle to steer) used simply as an illustration.
2.Another option is the
WeedBadger used for tillage in vinyards and orchards as the weed badger would cultivate deep and also
eliminate brush for you too. The Weed badger is a mounted implement that is either side mounted or rear mounted depending on the tractor horsepower.