I'm not sure why you would go all the way to Nowra, because I would take the Macquarie Pass from around Dapto to go to the Southern Highlands.
If you want to go to Nowra then you might as well stop off at the Fitzroy Falls (or one of the other falls around that area) on the way to Moss Vale, but beware - there are gravel roads in that area
To me going west and south/SW from Sydney are 2 separate trips.
Depending on when you start, and the season, it may be too dark to see anything by the time you get to Katoomba. Also you would need to pay the M7 toll to avoid wasting time. Wouldn't you rather spend the day walking around the places you drive to instead of sat inside the car?
I'm planning to do this late March, either on a Thursday or Friday. You do have a point. I've looked up places of interest along the Grand Pacific Drive, and there does seem to have plenty of places to do a quick pit-stop. Might even drop by Jervis Bay for the renowned white beach if we've time.