Andrew Luck's career completion percentage is 59.2%
You are making too big of a deal over completion percentage. You can get a great completion percentage by checking the ball down all the time. Yards per attempt is probably a better measure here to consider. That factors in both your completion percentage and the aggressiveness of the throw.
I gave my short list above somewhere early in this thread - from the never-been-a HC coordinator ranks, Frank Reich, and from ex-HCs Callahan, Smith, or Schwartz. I also mentioned Tom Cable, but NCF pointed out his history of abuse against women, so I would agree with NCF and remove that name for sure. I was unaware of that history.
I like Mike Smith. 66-46 record, 1-4 post season which needs improving (why Dungy was let go too, viewed as unable to get over the hump), and a defensive minded guy.
Those weren't recommendations, they were proving the point that veteran coaches, some with winning records even, and appearances in the Super Bowl, end up available every year, and can be had. And with Luck as a selling point, you'd think the Colts might be a place where a veteran HC could land.