If you look at the maps that are available, you will notice that Sabana Grande de Boy? is out in the middle of nowhere.
You can get there from the North only by using the new, toll, highway, as is mentioned in Option 2 above.
From the South, you can go to Santo Domingo and then find buses to Monte Plata and from Monte Plate to SGB. That whole area was sugar lands at one time. I would imagine that they are pretty abandoned today and poverty kind of rampant.
From the West, you can go to Cotui from La Vega or San Francisco de Macoris, and then on to Cevicos, and from Cevicos to Sabana Grande de Boy?, obviously lots of buses and lots of moving around, but a pretty interesting experience.
Let us know what you do.
HB