I found this series of YouTube videos compiled by Unknownmathew22 exploring different types of “dark ambient” music. I sort of stumbled into making dark ambient on accident, so I found this examination of different styles and subgenres helpful:

Dark Ambient, Example: Lustmord
Noise Ambient, Example: Aube
Clinical Ambient, Example: CircumLiver

There’s also some text descriptions of the different genres here.

Four more videos:

Isolationism, Example: LULL
Death Ambient, Example: Endvra
Ritual Ambient, Example: Endvra

Black Ambient, Example: Moëvöt, Aäkon Këëtrëh and Darkness Enshroud
Doom Ambient, Example: Nortt

Drone Ambient, Example: Troum
Deep Ambient, Example: Amon
Black Space Ambient, Example: Tangerine Dream

Neo-Classical Ambient, Example: Mauve Sideshow
Medieval Ambient, Example: Grabesmond
Martial Ambient, Example: Across The Rubicon