I think the best way to do it is to have lots of very broad brush npcs, which get painted with finer strokes as the players get to know them.
So an npc might be
Bonce Slopford, Baker, bald/sweaty, nasal voice.
That’s a memorable character in 7 words. If he never plays a part in the story – win! If he does play a part in the story, say by baking silver pennies into tiny buns then using them as sling-stones to defeat the Werewolf of Vampiretown – win win! Go Bonce you sweaty legend!
One trouble with a huge long backstory is that unless you’re a master actor it’s just not going to come across except as (tedious) guarded ambiguity. And if you are a master actor, then you can convey the impression of a huge backstory with an inflection or an expression.
Another one is that you’re closing off elements of the future story based on what you’ve already decided for the character.
I know that making characters is an entertaining game in its own right, but even if that’s the way you roll I’d strongly recommend slapping a seven word description on top of that once you’re done, because that’s who your characters are going to meet.