They rushed everything and made it seem incredible and unrealistic. I don’t doubt there’s a chance she is going to go all Mad Queen in the books but it will be gradual and more people will notice it, not just Varys. Bran being king I don’t understand but perhaps it will be better foreshadowed in the books.
Now, with the books the ending lines of each chapter hold importance. In Jon’s chapter when he meets Tyrion, the line refers to Tyrion’s shadow making him taller than usual and it says, “for a moment Tyrion seemed as tall as a king.” If Bran is in a wheelchair, Tyrion would be almost at a height with him as opposed to how tall he would be standing. This could be foreshadowing or it could mean Tyrion serves as a king briefly after Dany’s death before they choose a new king.
Read the books, people. You will enjoy them far more than the show, especially since the show left us with so many unanswered questions. The books won’t do that.