Could you find a child from the wasted North that could rival the intelegence of Shae? Probably not. But they could make good cannon fodder.
Well, if the Mangaecca exists anymore in human numbers this is really the only option. And Shaeonanra is an anomaly. He figured out how to do something that Mangaeccan Grandmasters of over two hundred years couldn't do.
The rest of the Mangaecca were originally derived from normal human stock. If anything, being brutal to begin with and using the IF as a tool of indoctrination is probably horrifyingly effective.
Lol - "You're of the Few. Come train with us and have power" followed by a nicely timed "oh, yeah, fyi, doing sorcery Marks you as Damned, here look at this"

.