Since it sounds like a 6 hour bus ride from Granada to Madrid, would it be quicker to take a bus to Malaga first (1.5 hrs according to Ricks guide) then the high speed train from Malaga to Madrid??? We could also hitchhike but they might take my beautiful wife and leave me by the roadside.
You can take the train (presently bus/train combination due to renovations of railways) from Granada to Madrid & the travel time will be around 4 hours.
EDIT: You may want to refer to this post for more information on railway disruptions around Granada:https://community.ricksteves.com/travel-forum/spain/summer-train-disruptions-on-granada-bobadilla-line-granada-madrid-granada-sevilla
I suppose your fastest option would be to fly from Granada to Madrid.