And even if you control 2 Bhaalspawn in your party their story is not the same... And you even don't know it until half of the second game because it's a part of the companion and main plots.
You may not like BG1/2's story and of course, 20 years after some things are way better in BG3 to play a role (tags, reactions to our race choice, to our answers,...).
But it's not relevant at all to the custom VS origin characters discussion anymore and no one ask this to change.
It wasn't only 20 years later, other cRPGs that came out back then (Fallout, Arcanum, Planescape Torment) had character reactivity towards choices made at character creation. This isn't some modern invention in gaming.
And this characer reactivity is how you make a character feel unique, in my opinion. A story telling me "oh wow, your character is so special" while it's pretty obvious this story also doesn't care for the most part about the character I've created? Doesn't make me feel like that character is unique, quite the opposite. It becomes clear that it's a story shaped without the protagonist, where you just tag along for the ride. That is why I disgaree that removing origin characters is somehow going to make Tav more interesting. It doesn't add or replace character reactivity. I like for example the custom backgrounds idea, because it would allow me to further differentiate between the Tavs that I create.