Again, I've never had a problem with 1st level preventing the inclusion of relevant backstory that is actual story with events with allies, foils, contacts, villains and mysteries. You can always have NPCs in the backstory show up and do something plot related. All it precludes is you being powerful, which I do recognize is a deal breaker for some people. But you could totally do Firefly where all the characters are 1st level to start and not already badasses without any real changes in the backstory.
I think "leveled" characters as starting characters works better for gritty campaigns than fantasy campaigns where "levelling" up is colored as a realistic increase in experience. For one thing, it works hard against the typical fairy tale tropes and bildungsroman that is the basis of so much fantasy literature to do that in fantasy. For another, too much backstory and too established of a life raises the question of why the character is suddenly going to make new best friends. For another, the levelling up in a gritty setting probably involved doing mundane things like going to college or working a trade that doesn't actually make for interesting story.
But as a general rule, you should always start the story when it is about to become interesting.
And also as a general rule, how much backstory is valuable to a campaign depends entirely on how many players you have. If you just have one player, the whole campaign is about their backstory and personal goals. If you have two or three players, then the campaign can be heavily influenced by each player character's personal goals and backstory. By the time you get up to six or eight PCs, backstory is secondary to the group goals.