might want to try /r/linguistics

I'm sure if you search that sub there is plenty already on this topic!

First, "twang" isn't a defined thing. A number of different dialects have, at various times, been referred to as having a "twang".

Second, the fact thaty they were from Europe isn't super-relevant. British accents were different in the past, and there was (and is!) regional variation in British English before and after American colonization. See here in the FAQ.

As for features specific to Southern US English, the Southern Shift (a defining feature of Southern US English) took place gradually during the 19th century (source: Labov's Atlas of North American English)