I don't think you can justify a lacking cast with "that's the story". Especially when it's about mind control that works on everyone just the same, including undead characters (which makes zero sense btw, they just wanted a vampire character to cater to Twilight fans and ignored all inconvenient logic).
It would actually make more sense to tadpole good characters that would 100% oppose you otherwise for a bigger power shift. Neutral and evil characters like Astarion, Wyll or Gale could be bought in exchange for fixing their situations, with no tadpoles required at all. Bit of a misstep right there, Larian. Lae'zel would do anything to impress Vlaakith, no tadpole required.
Larian went too far in making everyone so extra special and connected to the story. They completely omitted the backbone of any cast, the knight in shining armor, without any extravagant backstory or major condition. The down to earth good character that would have put all the evil and morally grey characters in context. The Ajantis or Edér of BG3. Or the light hearted adventurer who helps people they come across. The Coran of BG3, completely missing. Or how about the mercenary type who's not heavily invested in anything who's just trying to make their way, like Safana. A normal character who is cool simply because they are. There is no frame of reference. There's just a weird cast of edgy unusual characters who are competing who's the most edgy. With all of them being heavily involved in the story somehow against all odds.
And then of course the re-hashing of two old "good" companions from BG2. Which is a complete failure as well. The best way of doing justice to those characters would have been to NOT make them low level companions, again.
Decades of CRPGs sure have cemented a cliché cast of characters in some player brain. Next you are going to ask for a miner dwarf who hates elves, a bard who spends all of his or her time flirting or drinking and tries too hard to be the epitome of charisma, a timid and innocent female cleric healer and a human paladin with two neurons both being dedicated to fight evil.
Clever, but it's not about clichés.
It's about finding the right balance between ordinary and extreme. One can't exist without the other.
if Larian wrote Game of Thrones, every major character would be Melisandre. And it simply wouldn't work. You can only have so many dark secrets, alien origins, gods for lovers or magic time bombs put in your chest. Larian are completely oversaturating their setting with such things, and it will just feel like "whatever" in the end.
