Is the 85mm 1.8G worth it ?
I believe you got all the answers you need but I will repeat the important ones
If you dont know if the 85mm is going to help you then you probably dont need it.
50mm is not a portrait lens, especially not on full frame, yes its good for half body or full body but its pretty bad on head shots.
85mm is focal range pretty standard and classic for portrait that alone tells you its a good lens for this task.
Nikon 85mm 1.8G is considered to be one of the sharpest lenses in Nikon lens line, as far as I know its only second to the Nikon 85mm 1.4G which is almost 4 times more expensive.
BTW did you consider 70-200mm 2.8 which many portrait photographers (including me) use this lens for portraiture ?