The main reason that writers base most Harry/Fleur fics in GoF is because she hasn't met Bill yet, which allows for the relationship between Harry and Fleur to blossom. And if an author does start the fic later, they usually have already broken up Fleur and Bill, or just didn't have them get into a relationship. Now, the problem with your story is that not only are Fleur and Bill still in a relationship, they're engaged. This means that there's no possible way that you can start a realistic relationship between Harry and Fleur until that relationship ends. Harry is an incredibly pure hearted person, and his character would never get in a relationship with an engaged woman. Fleur also isn't the type of person to cheat on her fiance.

I can only see three ways to start their relationship, all of which are pretty off putting. You can begin deteriorating the relationship between Fleur and Bill, and eventually have them break up, then after a while, begin the relationship between Harry and Fleur. However, to do this well, you'd have to fill up many chapters with just constant tension between Fleur and Bill, until they eventually break up. The second option would be to simply kill off Bill. Killing off Bill would quickly end the relationship between them, and allow for Harry to comfort her in her time of need, jump starting their own romance. However, this isn't a good option since you're basically wasting a good character and vilifying Harry. And the third option is just as bad. You can throw out every bit of character develop Harry has ever had, and have him help Fleur cheat on Bill. Yeah, bad.

It just feels to me like you've written yourself into a hole. I can't really see a good way to start Harry and Fleur's romance.