Before I give my reaction, I want you to give my personal perspective when writing. So, when I'm writing fanfics, I want to stick it on what really happened in the main series. Like the tournaments and such things. But if that's unavoidable, then it can't be help. Now, I think too many shipping would be bad but I think they're just extra characters right? Because if it's not, it's going to be so hard to make especially you are using First POV. I think because there's so many characters here, 3rd POV is the easiest to make, but it depends what easier to you. Going back, in my stories, I really just want to focus on the main ship if it's only one. And if the ships that you put on the story is just extra, don't focus the story to them unless there's something important to them. (Example is the chapter when I focus to Sugawara because you know what happens).
Next up is the characteristics. I don't know with so many people, but I'm sick of the fanon characteristics of them. If you notice on my works (all of my works), I use the canon characteristics of them with slightly changed because I think it would be easier if you just put their original characteristics. Lastly is the wording. Nothing to concern about, if try to put * in some word that can be triggering. Ex. "F*ck" also, put a warning if there's some.
Overall, this is your first one so I won't judge and pressure you since I'm also new on making story. The main advice that I would give you is that stick to what really happens and their real characteristics.
That's all! I excited for you and for this book.