Crowing is an indication - try to process within 6-8 weeks of them starting to crow if you want to roast or grill them.
Wyandottes and Australorps are both relatively late-maturing birds, and both very good eating. Wyandottes in particular go through a very lanky phase in their late teens - 14-20 weeks - where they're not really worth processing. I always leave mine much later (OR process at about 12 weeks, when they have a chunky baby phase).
Wyandotte is about my favourite for standard-size eating birds, and I prefer to process them about the 24-28 week mark. They're crowing by then, but still tender enough to roast or grill, and are _delicious_. Australorp x Wyandotte would be about the same.
The ISA x Wyandotte may well mature more early because of the ISA influence - you might get away with 20 weeks.
Otherwise, if you end up processing later, they will be excellent for cooking into curries, stews, casseroles, stock, and so on. You can also strip the meat off and use it in stir-fries, even in older birds, or mince for various purposes.
I've just made coq au vin from 8-month-old cockerels and OMG, now I understand why the French have so many excellent chicken stew/casserole/slow-cooking recipes. It's superb, but you'd lose the real flavour and richness if you made it with commercial little chickens, poor things.