Flew three times with < 1yo in business to/from Australia. Check the bassinet requirements and if your little one will fit, book a bassinet. If infant won't fit, they won't allow it. They are likely going to be night flights to/from AU and mine slept pretty much 90% of the time. It was an A380 though.

The fourth time he was too big for the bassinet and it wasn't a whole lot comfortable.

Staying in any lounge for 9 hours before/after 15 hours in a plane with infant is suicidal. I'd book whatever I can at any price.

Etihad crew were absolutely amazing with us when he was awake on all flights. AMAZING!