Because QFF points cannot be transferred to any other airline's FF system, you are quite stuck, so to speak.
Using points, all you flights will have to be in the OW alliance, so if only Y is offered, have to take that.
There are some hotel points that are tranferrable to QFF, as well as credit cards spent, but if you have only got QFF, you have to use that.
Coming back via HKG in Y, if that is what you can see with CX, its the only choice, in OW without stumping more $ for comin back one way Y with TR (Scoot) or SQ.
Not sure what the ideal solution is tho.
Might have to pay one way Y coming back with SQ, not cheap, but the only way to get back to Aust with a bit of comfort, esp with bubs.
As mentioned on other replies, you can fly the ultra low TR from Europe (they currently (I think) fly from Germany & Greece) to SIN, then SQ from SIN to Aust.
Ultimately 440xx_ QFF points, is not really a lot of points, esp for 2.5 pax if you take an indepth look at it, and covid has made the flying situation worst, lots of airlines are still just coming back into full/partly full capacity.