There is absolutely no reason not to start the book with NaNoWriMo.
In a recent discussion I was having about the writing process, someone brought up the idea that there were two types of writers, those who write like architects, and those whose writing style mirrors a plant germinating out of a seed. The architectural style requires a great deal of planning and forethought, and the act of implementing involves adhering to and carrying out your previous plans (which is not to say that you shouldn't allow the structure of your piece to change based on revelations during your process.) The germinating seed begins with an idea, scene, image, place, or any other spark, and the writer writes out of that spark and finds the story and it's characters through the process of writing.
(This is a false dichotomy, and I'm not at all convinced that these are the only two styles of writing, and I feel like the idea that you can only be one or another is absurd, but they are two very common approaches to writing fiction which are worth considering and also pertinent to this question.)
NaNoWriMo does encourage the seed germination model of writing, but that shouldn't stop you from being an architect with your prose. The nice thing about writing out from an initial spark over a short period of time such as a month is that spark-writing has a far lower barrier to entry, making it a great model for someone who wants to write a first draft of a novel in a short amount of time. By choosing to plot out your model, you've given yourself more work before, and during, the writing process. You've already spent a great deal of time working on your world, characters, and plots, so once you begin, you will have a set of guidelines for how your plot and characters should progress. As you begin to write, you may find that your characters develop differently than you expected, and you might need to revise your plot because of these surprising characters. Changing the plot you've worked so hard on might slow down your writing, and generally deter you from completing NaNoWriMo. You may also face the problem of becoming bored with your story — since you know what's going to happen, you might feel less interested in writing compelling prose. Finally, being an architect incurs the problem of not necessarily knowing how long your piece will be. You may have written a plot which you think will take ~65k words. If you realize after a week that you're at ~20k words and only one tenth of the way through your plot, you might become discouraged by the goal of finishing NaNoWriMo in a month.
Despite all of those issues, I still encourage you to start your project with NaNoWriMo. It sounds like you have a well thought through idea for your novel, and there's no reason not to start the process of writing it with NaNoWriMo. All of the issues I've mentioned regarding the difficulties of preparation are valid, but remember that there are just as many problems with the seed germination process of writing. The latter involves far more rewriting, cutting of unimportant scenes, ideas, and tangents in subsequent drafts; the nice thing about these problems is that they still allow you to quickly churn out a first draft in a short amount of time. Many of the people who I know that write off of sparks end up rewriting their second draft completely from scratch based on the ideas and concepts that came out of their first drafting process. I know far fewer people who write from heavy outlines, but I'd imagine that their rewrites are generally less grand in scale (though I have no confirmation on this, and should spend some time finding out.)
Good luck on your novel!
