I'm taking a stab on it again, be ready for a rough ride.
Who your character is, why is he a blank slate. Your character is noone special or at least wasn't, just another Mind Flayer thrall. In fact they were the janitor cleaning up after all the messy experiments, and bad tasting brains. You were noone special, your life, your thoughts, your hopes and dreams null and void by the power of the Elder Brain. Yet brief sparks of cognition awakened things in you, when your masters psionic grasp slipped for just fractions of seconds. Then it happened 'Thrall 10562h please report to the impregnation room' yes you were called to serve the Elder Brain in a higher capacity, you were no a buffet, a mother. Your destiny was to become a brief meat suit for a hungry, hungry tadpole. A tadpole that desired a touch of divine, or a sprinkle of fiend, a dash of penaze, a pinch of Old One, a hint of rough and tumble, so you were callled. Then things got harry, when Sally and her dragon came storming out of a portal, the ship that you had kept cleaned for so long was now a battlefield. And you the newly appointed meatsuit, caretaker of an innocent hungry baby tadpole were finally free of your psionic prison.
Long story short even though you seem to be without a past, or with out ambition, D&D is about theatre of the mind at times. They can be who you want them to be, until Devs decide otherwise, their past, their present, and future is up to you to decide. They seem bland or blank slated go with this, or change it maybe they had an over bearing parent that forced them to act a certain way, or controlled them and now their free they don't know how to act, how to be themselves. Maybe they were simple, and that tadpole changed them, and now they are forced to deal with everything in a new light. Maybe their just simply unemotional, uncaring about the world. That they simply thing this is minorly annoying, and all these people whine to much, and all the want is to sit back read a book, and simply exist.
All those suggestions are valid ways to play and imagine your character, but I also feel as though what you're suggesting is letting Larian off for not handling the custom PC well. Your suggestions all are about making due with the mediocre approach Larian has taken. Lots of games give you blank slate characters. I keep referencing Pillars of Eternity, but really that's because they did the best job accomodating a blank slate protagonist while also giving players the chance to define their characters in game. At the very start of the game your character is a blank slate, with nothing defining them but a background and a nation of origin and the fact that they're travelling to a place where there is an offer for settlers to acquire cheap land. That automatically gives your character a grounding in the world, and then you have a chance to talk to someone and expand upon that in a conversation if you wish. That simple conversation enriched my roleplaying of that game by a truly massive degree.
And again, then you compare your custom PC to the vibrant, fully fleshed out origin characters, all with depth and history of their own. And in the context of them, it starts to feel like Larian just neglected the custom PC.