Trying to keep the Cornish X alive long enough to do something with them breeding wise will be tedious and time consuming, some of the lines of them sound a little more resilient than others but they were all still bred to be a "terminal cross" to gain that growth rate and they're not bred to live past those first couple of months.
Using Red Rangers or something along those lines would have less losses as they're growing out. In crossing them with the Australorps, you'll see some good results and some cull results, it's been my experiences that hybrids are fairly consistent that first generation and after that there is a higher cull rate in the 2nd and 3rd generation and you have to get even more selective in the breeding choices to move forward.
We raise Bresse and Marans (a meaty line of Marans, not that lean layer body type) and we see a dress weight from 3lb-4.5lbs around 16 weeks. At 14 weeks some of the Marans cockerels are at 2.5lbs and some are bigger. The bigger ones with good feather color coming in are held back as possible breeding candidates.
After a lot of dabbling with crosses and purebreds in various bloodlines, I've finally wrapped my mind around sticking with purebreds and relying on the culls for dinner. That way I can hatch more, which then provides more breeding choices, which then improves the flock faster. With improvements come a better price when I have good ones to part with.
Besides making Olive Eggers for fun, I've finally gotten the hybridizing out of my system.
Getting past that 2nd or 3rd generation of the higher cull rate in a project requires a lot of space, hatching a ton of them to find the better birds, the expense to feed, plus the time involved. In starting with good birds that meet your goals, you'll get somewhere faster.