While having an in-depth backstory is good - and essential for series and novels - from the way you speak of your writing I feel you may be limiting yourself.
I have the characters planned out so far, and I know where everything will go to.
Characters grow through the story. Good characters grow as a result of story events. Great characters grow independent of you, as they are part of the world you've created.
While the backstory sets the tone for the story, it shouldn't box it in.
When I began writing, I would make "profiles" for my characters: This is <PERSON>. 30 yo M. Easygoing, OCD, rides motorcycles. He's quiet unless it's a topic he enjoys then you can't shut him up. When he was 10 years old... you get the point. I always reached a place where I began to tailor the story to fit my perception of the character. The story became dry and stale, character backstory got in the way of plot, and nine-times-out-of-ten it went into the "Meh" folder. Once I adopted a more loose character profile my writing drastically improved. Now instead of thinking 'this is what <PERSON> will do', I ask myself 'what would <PERSON> do?'
The reader doesn't need to know every detail of the backstory of the world or the character. Good character development, narrative, and plot should give the reader all the tools necessary to fill in the gaps using their imagination. If certain backstory events are integral to the story - where the story would be worse off without them - I usually do one of two things:
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Use dialogue to reminisce or otherwise describe the backstory. Sometimes a little break from the action (I call them "campfire moments") for the characters to discuss the world is refreshing. In your case - there is plenty they could talk about. You can replace dialogue with things like newspaper snippets, journal entries, notes, etc. but this can get cliche fast so be careful.
2. A chapter that only takes place in the past. It could be a total non sequitur or involve a character from the main story but really make the most of this chapter because if you go this route you don't want to keep jumping back unless you're doing a story that takes place across multiple eras. Personally, I like using this method as the second chapter, especially in darker stories. First chapter drops you right into whatever "crapsack" the MC is in. Second chapter is a whole different vibe.
It's fantastic that you have so much backstory but I would recommend focusing on the current story first. Let descriptive writing, dialogue, and the other things we discussed earlier be your reader's conduit to the backstory. Let the reader gather the details as you take them on the adventure. Try not to spoon-feed.
One last thing. Don't be so certain that the events you've planned for your characters will be what actually happens to them! A well developed character can sometimes surprise even the author. 846