I know the feeling of having ZeroGPT say that what you write is AI, because I also had my works checked with a diverse array of percentage returned despite never having even touched AI. What I learned, however, is that ZeroGPT is not completely reliable: for example, it easily recognises text completely written by AI, but gives many false positives if it is human-written. I hope people using ZeroGPT before reading a fic know this too, and interpret the percentage not as "amount of AI-generated text" but as "probability that the text is AI-generated." (The difference is subtle, but it's there.)
Unfortunately, as writers we can't do much, and I would advise against editing the fic to make it sound less "AI-like"; it seems like a waste of time that may not even bring the desired result. Ultimately, I think that if a piece of writing is not genuine, the reader will notice: months ago, I came across a story that sounded strange right away, with lots of short, catchy sentences, so I had it checked and the returned rate was around 80%, which seemed plausible. So I'm sure people will notice the goodness of your story despite what ZeroGPT says, and I hope you'll still find the will to publish it.