Unfortunately, there's a great many problems with this, though they require some familiarity with Australia's history and geography to know why most of this is not feasible.
To start with, while there is a lot of gold in Australia (even today it's the world's second largest gold producer), the gold that's there is not suitable for an early, convenient discovery by any wandering Europeans. Viz, it's mostly a fair way inland and by no means obvious. It took almost 65 years after European colonisation of Australia before gold was discovered. While it's probably possible to shave a few decades off that, it's not likely that any random Europeans visiting Australia will discover gold and find it as a motivation for settlement.
For where the colonies are located, well, if I may quote myself based on what I wrote about Australia in the wiki many years ago... "Many ATL nations are carved out of parts of Australia which could realistically support only a population of 315 kangaroos and one lonely camel."
The parts of Australia which are colonisable in terms of early European colonists are, basically, the south-western corner, a couple of bits of South Australia (the Eyre Peninsula and a strip along the coast from Port Augusta to the Victorian border), Victoria - in the few good ports (particularly Port Phillip Bay, coastal New South Wales, and a few bits of Queensland's east coast - but the northern portions of that aren't much use except for sugar cane, and even then it really needs fertiliser to do well.
So for the "Portuguese horn" - I'm not sure whether you mean Cape York or Arnhem Land, but either way, not realistically happening. Cape York is cyclone alley and not useful in this era for much more than sugar cane and bananas (even with fertiliser). Arnhem Land is really hard to grow European crops on - it was tried and failed, several times, even those times when the fledging colonies weren't flattened by passing cyclones.
The northwestern coast seems promising, until you realise that the "average rainfall" figures on rainfall maps are misleading - sometimes that's 2 or 3 years worth of rainfall happening at once, and then nothing for 2-3 years. Even apart from that, the soil's extremely poor.
Realistically, whichever country gets to a place first and grabs the ports controls the place. It's easy to get Western Australia separate from the east, but given how far people have to sail, and the sailing routes (which require you to go south around the continent, and then north up the east coast) suggest 2 or 3 powers at most. Western Australia (southwest corner, not the Kimberley further north), a vaguely "southern coast" power, and possibly a separate east coast one.
Fur trappers? Perhaps, in a couple of places, but basically you've just wiped out the koala and the platypus.

Not so much over most of the rest of the continent.
For Aborigines as slaves, that would work about as well as the idea of using Native Americans as slaves worked in North America (not Mexico, but the east coast), i.e. forget about it. They were far too vulnerable to diseases for one thing, and for another were very familiar with surviving off the land, so could run away very easily.
In terms of Australia replacing the Americas for settlement... not going to happen once the Americas are known. The Americas are much, much closer for sailing time. Even in 1788, the First Fleet took
over nine months. In comparison, North America is just a hop, skip and a jump across the Atlantic. Much quicker, much cheaper, and the settlers are much less likely to die en route.