The best way is to cube it. You can cube yellows into Legendaries and reroll Legendaries.
The chance of getting a better rolled ancient or maybe primal this way is easier imho.
You should just need to farm DBs and Bounty mats (and essence too) you can go try it out and find what is more effective for your gameplay.
Either way, you farm. You can farm Rifts and Grifts and hope it will drop or you farm mats like crazy and cube the hell out of it.