I've done lots of flying Perth-Melb with bub, from a few months old. Try to get a bassinet but keep in mind they may not like being put down in it! Having a spare seat next to you is also wonderful in lieu of a bassinet IF you can get it- try to check in early and plead with the check in staff. If you have pillows (provided on board) and baby's own blanket/swaddle/bunnyrug and a toy or something familiar from their own bed you can lie them down on the spare seat for periods to give both of you some space to stretch out a little.
I was nodding along with this going "Yep get the bassinette if you can, yep, spare seat next to you, great idea, hang on what now?"

I'd never put an infant on a seat on their own. Sorry to be all negative about it but it just isn't worth the risk. Even in a seatbelt - adult or infant - they could come loose and there have been too many flights where I've hit unexpected turbulence for me to be willing to risk it. I was even uneasy with the bassinette for the same reason, but they at least have the mesh you can put over it.

I'm not an alarmist. I'm not a helicopter parent. I just don't take risks that I don't have to. You won't always get warning from pilot/flight crew that bumps are coming up and if you don't get the chance to grab them, well, it just isn't worth the risk.

Anyway, I highly agree with minimal hand luggage. Might seem ok while you are breezing through the airport with your nifty wheelaboard but if you don't have a second pair of hands to help you stow that bag or hold the bub, it can be tricky.

Definitely fly Qantas if you budget stretches. They are ace (mostly ).