I'm on the editorial team at First Round Review, and we actually just launched a new series aiming to share more stories about how different founders went about finding PMF. Check it out here: https://review.firstround.com/airtables-path-to-product-mark...
In terms of measuring vs. optimizing, I liked this advice from Sprig founder Ryan Glasgow: https://review.firstround.com/8-product-hurdles-every-founde...
He makes a distinction between validating whether you have PMF, but recommends using a different framework, specifically the Outcome-Driven Innovation method, for discovering or finding PMF in the first place.