Thank you, Morien - that's really helpful! I think I might split it, as you mention, and save some sort of wizard-king for later; I like the idea of the players having a bit more agency when they come to meet him in the Cornwall invasions and then their children meeting Cadwy's son later down the line. I was curious as to why the Cadwy in GPC seemed to struggle so much with Cornwall after giving Merlin pause in the Book of Uther, but that certainly explains it!
I might keep the idea of some sort of wizard-king on the back-burner for later...