Best farming techniques for ancient and primal

Running POJ and able to solo GR80 but I need more ancient items and they just don’t seem to drop, nor do they appear when spending blood shards or upgrading rares and reforging.

Is it better to keep running GRs to find stuff?
Multiple Nephlam Rifts (Go kill Rift boss then restart a new rift) or stay and keep killing stuff on a completed Nephlam Rift on Torment 16?

Continue to do bounties and collect horadric chests?

Note also I am a solo player, do more items come from plying in groups?

Ancients are 1 in 10 legendaries.
Primals are 1 in 400 legendaries.

The best way to increase your chance of ancients / primals is therefore to increase the number of legendaries you earn. The higher the GR level you complete, the more legendaries the guardian at the end drops, up to a point where it caps out around 12 or so. Run your GRs at whatever level you can comfortably complete them in 3-4 minutes each. This will maximise your legendaries per hour and, thus, your chances of ancients / primals.