So last night I finally completed GR70 and ended up at 510 paragon and the build I used was R6IK4 Boulder Toss. 800k sheet damage, 18mil toughness, and max fury 200+. Also did not have the 20% CDR breakpoint (only 16% maybe?). It was a lot more difficult than I thought it would be on a Barb.
So I started the night at 67. Then I slowly moved up constantly switching back and forth between CoE/Band of might and F/R. Basically if I failed a rift by not killing the guardian in time I would switch to F/R. If I kept dying I would switch to Band of Might. Probably not the best idea because each rift is very different. IIRC I used F/R to complete GR70. But man did it really test my reflexes and my eyes are not what they use to be. There were also times where I would charge and then poo would spawn directly under me as soon as the charge animation stopped. Sometimes it was an arcane orb and I would die instantly. Those were the times I calmly stood up and went downstairs for a little break.
As I got closer to beating 70, my anxiety started to go away. I became use to charging into the correct places and using my stacks more efficiently. I also learned what I can and cannot fight (this was probably the hardest to learn because I am stubborn). On the completion run I got to the rift boss with over 3 minutes left and 0 deaths. I felt a burden lift off my shoulders. Now I can work on my zbarb with my friends and hopefully get a sweet primal!