It is very easy at SYD to transfer to another airline because the domestic terminal is completely separate from the international so you have to make the transfer no matter what airline you arrive into SYD on. You arrive, you collect your bags, you go through customs, you turn left and go through a door and the transfer desks for both QF and VA are right there to re-check your bags and then you can hop on the bus over to the domestic terminal. It's easy.

If flying DL is a requirements, fly JFK - LAX - SYD and then book a separate flight on VA or QF to MEL and just accept no DL/ST benefits for that flight (it is short anyway -- it's really just the lounge access you give up if you have DL status).

I have done something similar on past trips when I flew UA instead of DL because of confirmable upgrade space on LAX - SYD and then switched to VA for the SYD - MEL flight to take advantage of my DL benefits on VA (prior to the partnership ending).

I would not seriously entertain the other options. I've flown them all (in various forms -- not all on DL/ST) and unless this is an award availability issue I can tell your from my experience connecting in Asia or connecting via Europe and then Asia is a nightmare. One time I flew RDU - ORD - NRT - SYD because PE was in policy for that routing but not for RDU - LAX - SYD and it was not worth it at all. Likewise on our honeymoon we flew JFK - FRA - SIN - SYD in SQ Suites and while the flights were amazing, it was very very long even in SQ Suites.