How to list a folders structure from Windows in a drop down menu

Hello, I’m trying to implement this proposal I made as a small addon:

The first step, I’d like to implement, is to list all the subfolders contained in the main folder (which path directory is assigned in the addon’s prefs) in a drop down menu prepended in the Sculpt Texture Panel, like this:

I tried copying the code of a similar addon, but it got too complicated.
I need something very simple.

def preview_sub_folders_categories(self, context):

    dirListing = os.listdir("L:\Alphas Sculpts") # How do I take the path from the addon prefs instead of writing it literally?
    cat_name = []
    for item in dirListing:
    preview_sub_folders_categories.indice = cat_name
    return [(name) for name in cat_name]

def sculpt_alphas_categories_prepend(self, context):
    layout = self.layout
    layout.prop(context.scene.preview_sub_folders_categories,"up_category", text = '') # How to make it appear in the Texture Panel?

Here’s the entire addon so far: (3.1 KB)

How can I achieve that? Thanks.