Earlier quoted context omitted.
Please don't use "High German" in English to mean (some perceived) "Standard German". It's bad (and incorrect) enough to use "Hochdeutsch" in German, but "High German" has a specific meaning in English: https://en.wikipedia.org/wiki/High_German_languages and the dialect spoken in Hannover is not in fact a High German one.
Wikipedia calls it Standard (High) German. To be more specific, I meant German Standard High German. https://en.m.wikipedia.org/wiki/Standard_German
And for whatever it's worth, the Auslautverhärtung you referred to in several places in this thread is also just a Northern regional thing: https://de.wikipedia.org/wiki/Auslautverh%C3%A4rtung / https://en.wikipedia.org/wiki/Final-obstruent_devoicing#Germ...
Here (eastern Austria) "Rad" and "Rat" (can) sound different.