Outdated and unnecessary.

Even if something were to somehow happen to where both Charles and William were no longer able to fulfill their duties before George came of age, the laws and traditions of the royal family would dictate an interregnum period. A regent would be appointed to exercise royal prerogative on George’s behalf until he came of age. 

And until 2011, the Royal Family was patrilineal. It was the first born prince that got priority rather than the first born child. If a first-born princess got a younger brother, she was automatically. The only reasons that Elizabeth became queen Regnant were that her uncle, Edward VIII, abdicated with no issue, and her father had three daughters and no sons. 

Australia being a member of the Commonwealth, the Monarchy hasn't done anything for us in decades, not directly anyway.
Post WW2 the monarchy underwent a lot of changes to scale their involvement back, which I thing was a positive thing in hindsight.... Empires are expensive to maintain.

We even had a national referendum to ditch the Monarchy and become a republic, but as a nation we actually loved the Queen so we voted to stay the course.
We don't pay the British anything or the Monarchy anything, the Governor general is elected on advice from our leaders to represent the King/Queen of the day, so the Crown really is just a figurehead that represents our past, rather than guides our future.

I wonder if we were to re-vote to become a Republic if it would pass while King Charles is in power? I feel his admiration isn't as significant as the Queen in today's society, but historical ties can be hard to break, especially amongst conservatives who are afraid of change.

Either way, I did receive a medal from the Queen for my efforts in the 2019 Bushfire season.

Governor-General Sir John Kerr apparently decided not to make public his dismissal of of Gough Whitlam as PM out of fear that Gough would contact the Queen and petition for the dismissal of Kerr. 

I am the king of the world.

Charles does not have the popularity his mother Elizabeth did.

It depends how long Charles is king, but I would not be surprised if at least some commonwealth nations leave and remove him as their head of state. 

Quite a few commonwealth nations did so with Queen Elizabeth. Most recent was Barbados, which renounced the monarchy in 2021, though they remain as a "republic within the Commonwealth." Conversely, Gabon and Togo recently became members. Gabon has no history as a British colony or protectorate. It was under French rule until 1960. Zimbabwe has been working towards reinstatement in the Commonwealth since the death of Robert Mugabe, who withdrew Zimbabwe from the Commonwealth in 2003 after their status was suspended a year earlier.

May your reign go on longer and healthier than the last guy:

My views on monarchism are varied.
I lean towards abolishing them all and finding them outdated. Saudi Arabia is an absolute monarchy and a very repressive country that influences the world in some pretty negative ways.
Limited to figurehead monarchies are more tolerable. The Emperor of Japan seems entirely ceremonial and a symbol of the people. He doesn't seem capable of doing his nation or the world any meaningful harm. The worst part would be the tens of billions of yen or so a year the taxpayers provide to maintain the monarchy. And that is one of the biggest arguments for abolishing monarchies.
Charles III's coronation cost a staggering 72 million pounds or about 90.7 million USD.

The Japanese Imperial Family is not treated with anywhere near the same pomp and circumstance that the British Royal Family is, thanks in part to MacArthur's actions during the U.S. occupation. Their means are relatively modest and a negligible expenditure for Japan. With that said, the British royals are a big tourist attraction and a source of the UK's soft power in the world. The Japanese ruling family, not so much. 

However, even Charles's coronation was relatively modest compared to the obscene amounts of money spent on Trump's inauguration, and Trump's inauguration was largely a walled garden for his oligarch buddies.