So I went through all my material music library and a bit through online stuff and I came to the conclusion I am not listening to anything fitting your criteria. My first intuition was right, my dark stuff is almost always rather on the brutal side (and often mixed up with less dark things, I love contrast within songs or albums).
I had to set on a few things that fit the darki(ish) criteria, probably more on the melancholic side of dark.
Let's start with Agalloch, as Minerrale also suggested them.
There are harsh vocals in some songs, but some other are free of these or pretty much.
Fits: dark, gloomy, melancholic. Smells of wood. COOOOOLD woods through. Chilly mist for sure.
I give you the only song that I remember has having mostly clean vocals. I'm sure there are more fitting ones (more moody, more gloomy), but as I was listing to the albums yesterday to choose a good one, I was always forgetting to keep an ear on the kind of vocals, haha. @minerrale would you have better suggestions?
The two next are less fitting but are of a melancholic, slightly darkish, contemplative mood; and these are two great bands with excellent singers, so I don't feel bad suggesting them even if a little off topic ^^
One of the voices that gave me some faith in 'popular' music when I was young (I'm so happy I did not remain a classical snob )
This one, depending on songs the singer can do anything from angelic voice to some of the most brutally aggressive vocals I have in my music library. But there are a good proportion of songs like these (no harshs at all):
In dark suggestions, I can't avoid placing my favorite opera composer. It's a delicate dark, but it's dark nonetheless, and melancholic, and creepy. Very creepy at times. This one is a ghost story, maybe a bit more fitting? That part is the first encounter with one of the ghosts. I love the foreboding mood coming little by little in the music, here. I could not find the scene alone, but the video should start at the right time, and it goes to about 21:00 minutes.
I'll stay in classical, and propose something that has NOTHING to do with the dark theme. It's because you said Southern Gothic, and that's the first thing that came to mind. It's very nostalgic, but happy nostalgic, and although there is incontestably a Southern vibe, it's one of the most universal pieces for me: it reminds me of Southern France, of Kuwait and of India! It has this generic, but great, reminiscence of any little quiet neighborhood in a warm, dusty place! This song is actually one of the first inspirations of my comic! Love this version with Leontyne Price:
Ok, that was not long, but I hope it will be somewhat helpful (even if it is helpful, to know what you don't want as suggestions for next time haha. I don't do the 'offended' thing )