This isn't even a casting thing.
As great as Chernobyl was, Mazin, it seems like, just doesn't get Ellie. No matter who you cast as Ellie, Mazin calls her incompetent so she was always going to be written this season as kind of dumb, even though the game and the first season makes it a point of showing that she's actually smart as fukk.
This season is full of people saving her or coming up with plans/strategy because she can't do it. It's her going against everyone's wishes, getting herself and/or others in trouble and needing to be saved and then scolded after. That's now how it was in the game.