Thinking about it, I don’t think anthology novel is the right word.
It either 5 or 6 chapters broken into 6 parts leading up to over 70k words. Each chapter focuses on a different character, but the chapters gradually connect with each other over the course of the story.
I don’t want to give any sort of advice as I don’t want to hinder your creativity, nor am I an expert. But I do want to recommend these books on plot and structure:
John Truby’s The Anatomy of Story
James Scott Bell’s Plot and Structure
Black Snyder’s Save the Cat
Jessica Brody’s Save the Cat Writes a Novel
Sol Stein’s On Writing
I don’t want to insinuate that plotting is the end-all-be-all for keeping a novel from becoming a series. But checking out these methods and applying the one that speaks to you the most–with some creative variations of your own–might be a good solution to this problem. Whether or not you are an architect or a gardener, plotting and outlining can benefit any writer. Even if in the end you do something completely different from your outline, it still forces you to think about your plot as it relates to your character’s development, and that’s the largest determining factor when it comes to the length of your story.