Perhaps a brush-control firm which uses goats to graze on understory plants can help with the blackberries.
The owner can tell you whether the goats eat trees (I don't think they do). I've seen goats used for clearing dense brush next to the freeways and on steep hillsides in the Bay Area.
And they leave little..."bits"
of fertilizer behind, too...
Sunset Zone 17, USDA Zone 9