G’day Stuart,
My Wife and I went during the start of DEC which was good as it was the tail end of the busy season and it was quite at the resort we stayed at. If you take your Wife I would recommend the staying on the island as she will enjoy her time more. We did 2-3 dives each day with an occasional scuba dive on some shallow wrecks. You will be able to visit many wrecks with your quals and yes Nitrox is a good idea. I wish to return with more advanced quals so I get down to see the two Destroyers both sitting at around 60m. Either way you HAVE to vist this place as the wrecks are decaying and its unbelievably amazing!
Jeff