If Gerwig wants to stay more true to the source, she needs to understand CS Lewis's theology. The Chronicles of Narnia are children's tales (fantasy) woven in with his theology and his personal experience of conversion to Christianity. As for the Lion, the Witch and the Wardrobe, you mentioned Arthurian legend, which it does contain some, it is an allegory of Christ and His death on the cross to save sinners.

I'm not holding out hope that Netflix will stay true to the Christian elements of the stories, but it sure would be nice to see.