Hello, this is an interesting question. If I might add some historical context.
I do know for certain that there was debate about establishing the British dominions as independent kingdoms with younger sons as their first monarchs. Certainly in the C19th & possibly in the early C20th. I don’t have sources to hand to refer to but I have definitely read about this idea before somewhere.
I think with the growth of rival great powers like Germany & their new empires this idea was not taken seriously because it made sense to have one monarch that everyone could be loyal to. This would ensure imperial unity.
Joseph Chamberlain who was colonial secretary suggested that Edward VII be titled “King of Great Britain & Ireland & of Greater Britain beyond the seas”. Chamberlain was also an advocate of federating the dominions & self-governing colonies & creating a United Kingdom of Greater Britain. Lord Roseberry suggested “King of the Britons beyond the seas”.
In the end the decision was made to call Edward king of the Uk & “the British Dominions beyond the seas”. Dominions here confusingly meaning all British possessions as well as the then two dominions of Canada & Australia. George VI was the last monarch to have that title. Elizabeth’s title reflected the political realities of a different age.
As I’ve mentioned the context in the late C19th & early C20th is one of great power rivalry & an arms race in Europe. Imperial Germany was constructing a vast navy & building a colonial empire. This concerned the British so much that alliances were formed with traditional rivals France & Russia. The stage was set for the carnage of WW1. So the last thing the British wanted was a weakening of their own empire. One king served as a symbol of unity throughout the empire. When you look at the sacrifices made by dominion troops in WW1 you can see how vital they were to British victory.
Into more modern times it was these same sacrifices that accelerated the development of the dominions as nations independent of Britain (they signed the Treaty of Versailles for instance) as well as their own national identities distinct from just being “overseas Britons”. This was formally recognised by the Statute of Westminster in 1931. By this time it was just simply too late for any idea of British princes to be made kings of the dominions. It was an idea that belonged to a different age. The reality was a choice between a shared crown or a republic. That remains the choice now of course.
Yes, I agree. If any of them were to go through the trouble of the constitutional amendment process to end the shared Crown, it would not be to replace it with a local king, but rather to have a republic, which will probably still happen in many realms in the next 50 years or so, maybe even earlier.
It is also interesting how the "dominions" as they were still called in the Statute of Westminster became the "realms" under Elizabeth II and how that was reflected in the royal titles and styles. It may look like an inconsequential change, but is meaningful in itself.
As for the history of how the Dominions came to be, you are probably more knowledgeable than I am, but my impression is that it was mostly a pragmatic decision. Maintaining a large, multinational empire was costly and complicated. So why not have the most "civilized" parts of the Empire mantain and pay for their own armies, courts, civil service, parliaments, and governments? It would appease the local demands for autonomy, thus preventing the repetition of movements like the American Revolution and, at the same time, keep them tied to the Empire, which could always use the Dominions' assets when needed (as in World War I for example). A win-win situation.
Of course, "civilized" meant not only capable of self-sustainability (i.e. viable as semiautonomous or autonomous unit), but also, for the Victorians and Edwardians, it meant having a transplanted white European settler society deemed capable of self-government under the British parliamentary institutions and Crown like free Englishmen.
Thus India, for example, despite also having its own army and civil service, was ruled by a Viceroy with instructions directly from London and no responsible government and, in the "white Dominions", natives were excluded from participation in state institutions. South Africa normally stands out along those latter lines because whites never rose above 20 % of the population there and inequality was publicly evident, but people forget that, in context, it also happened in Australia, Canada or New Zealand.
