Interesting itinerary. You are certainly getting your money's worth on the Discovery Pass.
I note you avoid any overnight travel by catching the Casino XPT and coach to Brisbane, but it will be an early start on the XPT back from Brisbane.
By travelling on most trains for the full journey there and back you make the overall itinerary longer, which I guess was the intention. You could, for example, have got off the train coming back from Griffith at Junee, then caught the Melbourne XPT from there, and covered Broken Hill in the second week. Your itinerary covers the Sydney-Goulburn line six times, so you will be seeing some of the same scenery numerous times.
Hope it is a great experience.