If QF are going to make these extra long haul flights work, they will need to choose the aircraft with the best economics. Considering the size of the potential market, I suspect that will be the 777X range of aircraft.
That's certainly a possibility, but we need to keep certain things in mind. A post-PS Qantas will have two specific subsets of premium routes:
1. The Project Sunrise Routes
(Provisionally, I will say these will be MEL/SYD - JFK/LHR, and SYD - CDG/FRA)
2. The Pacific Trunk Routes
(These are SYD/MEL - HKG/HND/LAX/SFO/SIN)
All of these routes do/will have First Class demand as well as substantial business and premium economy demand. The reason that I am not putting the DFW flights into these categories is that DFW is mostly connecting traffic, and the A380 flies that route because its the only jet QF have (at the moment) capable of servicing it... when PS is operational, I suspect DFW will be downgraded to 787-9s from Sydney and perhaps Brisbane.
Now that's out of the way, let's get to the analysis.
At least in theory, the 777-8 looks ideal for route subset 1, and the 777-9 looks ideal for route subset 2. But the problems here are that the number of 777-8s required would probably be quite low (thus increasing costs for QF), and
the 777-8 has been delayed.
So either QF delays Project Sunrise and goes all 777-8 (which is Too Much Plane for the Pacific Trunk Routes unless QF need/want that cargo capacity) OR QF temporarily runs PS flights with a very lightly loaded 777-9s, perhaps until the 777-8 becomes available. Which would require a sweet incentive package from Boeing.
The A350-1000 may simply be the more efficient solution for QF. Its roughly the size of the 777-8, but would have better economics (and still quite a bit of useful cargo capacity if QF need/want that) on the Pacific Trunk Routes. And if it allows a single fleet, that could save a lot of money for QF. Whilst its true that the A350-1000 in QF's config would be pretty low capacity (I'm thinking 270-280 seats, with about 81 to 85 of those being First/Business/Premium), the lower capacity would likely mean increased yields and possibly higher frequencies on certain routes.
Not only that but the A350-1000 isn't dealing with the delays the 777X is.
If Airbus can do relatively simple customizations (more fuel/extra MTOW) to get the A350-1000's performance up to the level Qantas requires presuming a low-density cabin, the A350 seems like the more streamlined, lower-risk option, and in addition isn't subject to delays. Its also consistent with the reality of fragmentation.