I'm surprised you don't like it. For me personal plots and relationships are the most interesting part of playing pen & paper RPG's. It makes the game much more complex and emotionally involving. I'm so grateful for pre-gens stories (especially for Astarion's, it's the pop cultural plot I've been waiting for very long time). To be honest, I have a theory that maybe all of these characters were chosen because of their unique aspects - maybe mind flayers experimented on them to create more powerful illithids or something like that?
Plus I guess everything happens so quickly because Act I is an exposition. You have to discover your party members' secrets to enjoy their progress later.
I personally think revealing that these characters were chosen to become carriers for non-transformative tadpoles because of their unique situations, would go a long way to help iron things out. However I do have a secondary issue... in that, yes... I do like personal plots in pen and paper RPGs... I do not like it when someone has adventures that a level 20 wizard would have, and them to join my party... not even knowing magic missile. Gale has an artifact level magic item on him, a huge history, has had sex with god herself... and its like "pew tiny bolt of fire"
nobody in this group should be level 1, unless the tadpole has already eaten our brains and the tadpole just thinks its the people whose brain they have eaten...
If anyone came to my table with these backstories, except maybe shadowhearts... I would say "no, dial it back a little, you are level one"
