some basic background of Shar would benefit the player base before Shadowhearts reveal.
Bingo.
This is the problem when Larian throws in too many ingredients right from the start.
Selûne vs Shar, Mind Flayers vs. Githyanki and Red Dragons, Nautiloids, tadpoles, the Nine Hells and Devils, vampires with Astarion and Cazador, the mysterious Cult and the Absolute, Jergal and Withers.....
They can't possibly set the stage so that the player has basic knowledge of all that. But Larian threw everything at the wall to see what sticks.
BG1 was much more focused on the Lord of Murder when it started slowly unveiling the mystery bit by bit. In BG3 you're in a huge theme park and all of the above is just poured on you.