I see no evidence that making tests longer will optimize for the variables that matter in academic performance (which are not 1-dimensional anyway) as opposed to simply optimizing for people who have poured resources into test preparation.

You seem to be coming from the perspective that testing must be the best metric, and then trying to develop a methodology based on that assumption. Purely as a thought exercise: it might be helpful to start from the assumption that testing isn’t an option, and think about what you might do instead.