I don't mind it being deep, just don't fill it with your actions and deeds. A big part of fun for TTRPGs is 'play away from the table', which for the players is typically making art, backstory or builds for current or future characters. Most long backstories I read don't invalidate a level 1 character but mostly explore values, just as my real life story could be as deep as I choose to write it and I'd not even have the skills to be level 1.

My suggestion is:

• Get people together for a session 0. Only pitch the campaign and tone then, if not construct it collaboratively too.

• Hand out pieces of paper or card face down, have each player take 1, and ensure there is one between each player. These cards say Love, ally, rival, or enemy.

• Explain that players should make an NPC for their backstory that matches this word, and should make a shared NPC with the person next to them based on the card between them.

• Now let them take another card of their choice. They can either make another NPC with this, or use it to make the relationship to one of their shared NPCs asymmetrical.

• They can design their NPCs and backstory now or before session 1, up to them.

Finally, explore what the players can choose to do to contribute between sessions to the game. If they don't do anything, that's fine, but they should have a way to meaningful contribute to something. Typically I encourage world building and cultural lore, such as unique foods and why that has a thematic resonance.

This is hard to structure, I had a player who was a former forever DM, who played a knowledgeable librarian in a former monster hunter guild. I asked her to make some monster statblocks, as she'd know them inside and out in character.

My advice to players:

Make your backstory show that your character has done no huge deeds yet, and most importantly, have everything that matters in it revolve around NPCs. Not just is this the best drama, but NPCs can move, join factions, be redeemed, betray you, die and everything else.

• That cost halfling village you design that perfectly exemplifies your character, but will never be seen in this urban campaign halfway across the continent? Make the most important part of it the mayor's daughter who happens to be your childhood friend.

• The strange necklace that made you stronger but more angry when you wore it? The final time you saw it was when your brother stormed out of your co-owned business after a bitter argument.

• The lord who helped you smuggle your liquor into the city? That's the same lord that wrongfully imprisoned the player character next to you.

One of my favourite scenes from a campaign came when a player, after spending a session getting the chance to meet with a resistance leader, turned to the others and said "this is my ex-wife". That whole dynamic was interesting too, as both had come from a warrior culture and initially parted due to neither being the "strong warrior", now both trying to fight against that same faction a decade later.

My all time favourite NPC was a talented tailor in an urban campaign, who owed one player character a favour and was generally fond of them all. Nothing like the party having a go to guy for fancy or silly outfit amendments.