The Monarch retains the power to veto any legislation, appoint/dismiss a Minister including the PM, prorogue/dismiss parliament as a whole, and technically all foreign relations treaties, declare war etc. but all this is done "on the advice of" a minister.

However, as an anti-monarchist I'd like to point out that for a couple centuries the Monarch has never once had a piece of legislation put before them that they felt the need to veto.

So how much power the Monarch retains is really a mystery, because it could be as simple as someone from the Monarch's office going "he ain't signing that" and instead of potentially introducing a constitutional crisis they torpedo the legislation.

The interesting thing to me is still how the Monarchy kowtowed to Trump, so given they have full authority to enter treaties/deals/pacts with foreign nations, what that could potentially entail.

In Australia, the Governor General is, constitutionally, the representative of the Crown, which the King is also. The GG in Australia can lawfully dissolve parliament under certain conditions, I think. Has been done in the past, anyway.

I expect that the King of the UK can dissolve parliament. But that's just a distant observer's speculation, of course.

And he used to own the U.S.A. until trump came along.

Ah, but what you don't know it seems, is that the Monarch is not 'The Crown'. The monarch is the embodiment of the Crown, and it's representative, but in British political history, the authority of the monarch to act has been limited and restricted, etc. The Crown is like the sovereignty of the country. But this doesn't mean that the monarch can do what he/she wants.

It's an important concept, as far as I know (admittedly limited - not a constitutional scholar).

But the Crown is the institution, the sovereignty. That sovereignty is exercised through government institutions.

This is the foundation of the modern United Kingdom, that took a leap forward with the Magna Carta and then another with the 1689 Bill of Rights.

In fact, much of the United States constitutional foundation builds on the UK system, but takes it to another level. In the US, the sovereignty of the nation is embodied in 'the People'. In the UK, it is in 'the Crown'. But Charles is not the Crown. For example, he has to take oaths.

The problem is not so much the form of the government. It's the corruption that infiltrates and abuses it. Corrupt presidents in the USA for decades have horribly gutted the country and caused a plethora as problems. In the UK, the Royal Family is or has (always) been corrupt, but some would have been better than others. But corruption in the Monarchy as well as in the executive is likewise a problem.

Both the United States and the UK have three branches - legislative, executive and judiciary.

Corruption is the problem.

And no, Charles doesn't 'own' England, Canada Australia or New Zealand. The Crown does.

They simply outsourced slavery.

With all due respect, this is an incredibly simplistic view of the UK system.

And no, 'technically' the King does NOT select the government. Like the US, the UK system of government evolved as a system of law and order. The King, or Monarch, is as subject to the (UK) law as Presidents Bush, Clinton, Bush, Obama and Trump are.

But, let me ask you. Are Bush and Clinton really subject to the law? If not, why not? Is the problem here the US constitution? Or something else?

The basis for the US constitution AND the UK constitution is the same established foundation of common law. They differ, but there is a LOT more in common than there are differences.