for the sake of releasing the fanfiction plot bunnies that run around pooping all over the landscape of my mind.

I have found two things effective for this:

The first is writing out story notes / the passages in question longhand on a pad of paper I keep near my bed as I'm falling asleep. (You can also write out general notes and reminders to get them out of your head.)

The second is being willing to just write the sections of the story that are interesting / currently in my head, without worrying about connecting them to other things or making it a full story. If I later decide that there's enough for a full story, I can stitch it together; if not, just getting out the part that wants out will give me the mental piece of having expelled it.