King Charles III? For sure. Why not?
Constitutional monarchy is a brilliant invention from a national viewpoint. It's a very stable system with an architecture of trust and deference built in, which would make it hard for a megalomaniac to subvert and corrupt the government or undermine the rule of law.
I think a structure of this sort or something like it is necessary for the protection of a democracy. You want an architecture at the top which is non-partisan to make sure the system itself is preserved regardless of whoever is making laws and executing the decisions of government.
America has a written constitution and separation of powers to seek to achieve the same purpose. But we are seeing how difficult that is to maintain.
Basically, the office of a constitutional monarch detaches the grandeur, ceremony and authority of sovereignty from the execution of government. So you can support the monarch and through them the nation itself, regardless of whoever is in power; and know that that person guarantees the fact that your party will get its chance to throw the incumbent one out.
It diminishes the office of Prime Minister, which is a good thing. PM is a partisan role. There's no harm done to the constitutional structure by criticising the person running the cabinet. In fact, it directs the popular will in a direction one wants it to.
I accept the criticism that it is strange to employ a family to guarantee a democracy. But in my view, paradox is part of the fabric of nature. So I embrace it.
But I don't envy the poor individual who gets stuck with the job. Poor Charles!
I agree Harry doesn't seem ambitious for the kingship. But he is very much committed to his role in the service of the monarch.