Tax anything above 1 billion at 100%. If that isn't realized gains do not allow them to borrow against it. No loop holes.
The US and others managed to set a global minimum coorperate tax so this is doable.
The point is NOT to take all of your money away, but to force you to RE-INVEST the money into growth and your employees. Otherwise you are just inviting schemes like stock buybacks and dark money flow into Washington DC, to make all of that legal.
I don't want billionaires running charities deciding who gets what. I want elected governments to do that. (yes I know the US government is wasteful and incompetent but not all governments are). The wealth these people have acquired is because of the public and the public should benefit from the excess.