I would say with a lens of this size, a tripod always helps. At least you don't have to hold all that equipment with your muscles. I agree that the picture is sharp, but noisy, and the focus is not completely where I would want it to be. Focusing on anything at f/1.4 is a challenge, and at the resolution of the D850 and such shallow depth of fields, I would not consider a person stationary by any means, even if they tried to stay in one spot. Just breathing normally will produce enough movement that things will move out of focus. So 1/60s shutter speed would be too slow too, even from a tripod, unless you really are photographing a steady subject, which, as we established, a human being is not. That being said, I think you could get away with around 1/250 or so, allowing you to drop the ISO a few stops, reducing the noise on the final image. Getting more light and stopping down slightly would help even more.