I've done the far north itinerary, the Cod Hole itinerary, the Taka GBR 3 day itinerary, and the Yolanga on separate trips. My honest advice is that GBR diving is Meh at best, and if I had any advice to give it would be to fly from Cairns to Madang PNG and dive there. If your heart is set on diving the GBR, then wait for a Mike Ball slot to open up, because being on a Mike Ball boat is worth the mediocre diving, but if you are just going for amazing diving, you won't find it on the Great Barrier Reef. It is worth changing your itinerary to go someplace awesome, and there is awesome diving in that part of the world, but in my opinion, the Great Barrier Reef isn't it.
The OP isn't even certified yet. From memory, Madang isn't exactly what I would call an appropriate destination for an inexperienced diver.