Higher level, Fukuyamas political order series does a great deep dive into these kinds of topics, really blew my mind, and made many archaic seeming political structures make far more intuitive sense to me afterwards.
https://en.wikipedia.org/wiki/The_Origins_of_Political_Order
https://en.wikipedia.org/wiki/Political_Order_and_Political_...
By that time too the men serving in the legions were increasingly locals and not Roman or even from Italy.