Maybe the problem is that you are trying to hire a PhD (whose entire value proposition is the notion of a complex, well-thought-out thesis) and you are trying to shoehorn them into the agile workflow, where people produce potentially crappy solutions (albeit something) and quickly iterate.

The whole point of a PhD is to NOT approach problems like that. Someone with a PhD has literally been trained to approach problems in a methodical, thoughtful manner, writing a thesis on the singular topic over a long period of time.

Is the problem here a fundamental problem with the PhD manner of approaching problems, or is it a problem with your startup mentality and hiring processes?