I'd keep Big Mike. Dak is done. A statue in the pocket.
Holds the ball too long.
By the way, stating that Rush gets the ball out quicker than Dak is not equal to saying Rush should be the permanent starter. Yet it is a point that's been made by every commentator that's done a Cooper Rush game.
So yeah, keep the HC and find him a QB. There’s a retread every year that resurrects his career. Mayfield, Darnold, Geno Smith, Russell Wilson, etc. Then draft a QB often, every draft if that's what it takes.