When George Lucas wrote Star Wars, it was 9 parts long, but he zeroed in on the "important" part of the story, which were the 3 parts pertaining to Luke Skywalker and his journey. What the audience needed to know about his past and the Jedi order and the Sith (who wasn't even called that...) was learned in the course of the story.
Make your book like that. Kill the infodump, and start with Ch. 10 if Ch. 10 is where your MC comes. You call him the "main" character for a reason - don't let the others rob him of that status by hijacking his story.
Well said. Expanding on Star Wars, I recall hearing something about it.
Originally, Lucas wanted to do the first three films first. (They weren't anything like the movies that came out this decade.) Someone told him "Nobody wants to watch three movies connected only by a couple of robots before they get to see the interesting stuff." He wisely agreed and started with Episode 4. (And because of what he learned with Episodes 4-6, his later Episodes 1-3 were much better than they probably would have been, even if I didn't personally like them so much.)
Also, almost nobody reads the Silmarillion before reading The Hobbit and Lord of the Rings. That's because the Silmarillion is boring, and nobody cares about it unless they get really attached to the setting due to interesting characters like Bilbo and Frodo.
Here's another example. Nobody would have watched the Underworld movies if they'd started with Underworld 3, which is the backstory movie set hundreds of years before the other two. The series would have flopped. Underworld 3 is a good movie, but it benefits from emotional investment. (On the other hand, it's important that it's a self-contained story, and you don't have to see the other two to like it.)
EDIT: One more. David Eddings wrote two 5-book series about a few years' worth of conflicts in a certain world. After that, he wrote two more books, told from the perspective of immortals who'd lived in the setting. Those books told all the backstory, nearly stretching to the creation of the world. Nobody would have ever published those books if the other 10 books hadn't sold so well, and those had been the first novels he ever submitted.
Nobody's going to want to know about your character's ancestors unless they like your character. That means introducing the character first, then introducing the ancestors. (Possibly through indirect references or later books.) You've already gotten to know your character, so you'll be interested, but remember your readers haven't. They have no way of knowing up front that there's a payoff for reading about all these other people.
If you've absolutely got to do it in chronological order, I'd suggest treating it as an anthology of several different novellas, each with different MCs. Also, I'd suggest publishing a few successful novels first so that a publisher will give this a chance. It just rarely works anymore. I'm sorry to be a downer.
But hey, if you start with the main character and use your knowledge of the backstory to write an awesome story, it could still grab all sorts of reader attention. Once you have their attention, you can tell the backstory.