I recall that famed southern writer, Flannery O’Connor opined several decades ago that ultimately the greatest source of conversions would be in the South.
And I agree with you that part of the reason is that religion is taken seriously in the South. I’m not in the Deep South. I’m in the southern Missouri Ozarks which is a subset of southern culture, rather similar to that of the Smokies and southern Appalachians (which is where virtually all of our original settlers came from).
We get a lot of conversions, mostly from the more Fundamentalist groups. Another thing O’Connor said was that southern Fundamentalists would be surprised to learn that they share more with Catholicism than they do with mainline Protestantism. And I think that’s true too.
So, while I do think a lot of growth in the Church in the South is due to influx of Yankees in some places, and influx of Hispanics in others, I think the conversion rate is also picking up.