I think !shortstories@literature.cafe would be a good place for it. The community sidebar says your own stories are welcome. You might want to add that you’re specifically looking for feedback

!creepypasta@lemmy.world is a decent option for horror stories. Though I heavily recommend looking outside of Lemmy. There simply aren’t many readers here.

Edit: and !lemmyscareyou@lemmy.world

• !imadethis@lemm.ee
• !chat@literature.cafe
• !books@lemmy.world

Let me know if you find a place. I would be interested in reading it.

There’s a community called “writing prompts” that I’ve seen in the Everything feed. Maybe check that out?

Royalroad forums

There’s one place that comes to mind.