I had to do the same not long ago - what i did was ask in general chat for some helpfull souls - to leech some txiii rifts / greater 70-80 spam... for just a few rifts, till i had just a bit of gear
Then i started botting like T4 rifts, collecting a ton of greaterrift keys stones, and once i had full pestilence set - i set it up after the pestilence guide in here - and started running 70s and then worked my way from there... now im clearing 95s in 6-8 mins - hoping to soon increase to GR 100s =)
But my best advice, is do normal rifts now, since u will need the keys later anyways - and it will save you time at that point, to have banked a ton of keys - gl!