Have you tried places like Royal Road, Honeyfeed, Wattpad, and other places like that? I don't know if they have dedicated forums since I've never checked, but I see a lot of discussions in comment threads under the stories posted there. And it seems to be a well known practice of writers helping/promoting other writers, with cross discussion on each other's stories and encouragement and whatnot.

A lot of writers who have channels on Youtube also probably have their own Discords where they invite fans and fellow writers to talk about things, but I find channels like that to be of VERY inconsistent quality most of the time so you might not find "good" communities there.