I wouldn't go as far as to call Wyll weak minded. Not everyone needs to be a Karlach cringe alpha just blurting things out left and right as the emotions take her. He has his strong opinions. What's missing is the content. He seems the most normal cast member to me personality wise, in terms of who you'd meet in real life in a normal setting, not some weirdo otaku convention or Tumblr community. He has his morals, he comments on other companion's situations, and he has a rather healthy evaluation of his pact with Mizora and of his interaction with his father. In fact, for how Wyll's father behaved, Wyll has a pretty impressively mature take on things.
Shadowheart can at times be normal, but then she's also a schizophrenic in writing. While commenting on Karlach's scene with Gortash where she's devastated she's gonna burn out from the engine, Shadowheart first has a sad face commenting on the cruelty of the situation, then immediately her face turns to a smirk and says we're all bound to die anyways. Bonkers dialogue. Astarion is always an insincere ass to anyone but himself. Gale would be another of the more normal characters if he wasn't constantly going on about his mommy issues with Mystra, and the final normal character would be Jaheira. Halsin could have been normal if not for the forced carcicature of him as a polyamorous horny bear the writers ruined his character with.
You don't need to make characters weird or caricatures to make them interesting.
I totally agree about Wyll, he has a very realistic grip about his situation and a very down to earth character.
I would include Lae'zel among the pretty normal characters. She is a typical githyanki girl, who gets her beliefs shaken, is never insincere and very open about her intentions.
