There's no specific way to farm DB's. But there are a few ways to make it allot quicker; as you said craft the Sage set and do it on the highest difficulty possible.
And then try to make a speed farm build with the character you have. There's no build which is the best, every class has a good speed farm variant build. Just pick the one you enjoy the most, not which one is the "quickest".
And for your last question: I'm also a big LeapQuake barb player, and I never changed my skill from Ancient Spear - Boulder Toss to Seismis Slam.
I don't like the synergy and Ancient Spear - Boulder Toss works fine and deals shitloads of damage.
If you're going Ancient Spear - Boulder Toss, use Animosity cuz of the rune: the more fury you have the more damage it deals