the way Etihad release their award is very unpredictable. They seem to come in waves, sometimes very generous and you can book 4+ seats, sometimes absolutely nothing. It is not like CX that seem to constantly release any unsold seat at last minute.
But looking at some US experience, they do release some last minute award spaces even the apartments. But that’s just some very recent trend and we don’t know if that can be replicated in business class to Australia. But bear in mind that F tend to have more unsold seats than J.
TBH, I don’t have any experience in last minute booking as I just don’t like uncertainty. I always book way in advance. But I can understand your situation where you want to travel in group and currently there isn’t enough award seats. I guess it’s just like any principles in point game, you need flexibility. So if you have to travel together, then book the rest in Y. If no need to travel together, consider travel on a day before or after if J is available. Then if later on more J award space come up, you can always change. If they don’t come up, at least you can still get to your destination.
You need to have this back up plan. Cuz even if EY do release last minute award space, what if your particular flight is full and there is no seat to release? In my experience flying EY J, I found the cabin fairly full with hardly any spare seat. So whether they were all seats sold out, or they were the people who applied upgrades (or even bid for upgrades), or they snatched last minute award seats, I have no idea. But I can say I kept my eyes on my flight to see if there was last minute award space being released, I can say there was none.