One of the themes of Stormlight really seems to be—it is never too late to become a better person. I love how you described flashback Venli's personality as cowardly, resentful, jealous and quiet because it's so true. She doesn't seem to have many redeeming qualities...yet when confronted with the results of her actions, and encouraged by Timbre, she does change for the better. And "better" doesn't just mean she turns into Eshonai 2.0, someone brave and outgoing etc. Venli is still Venli, afraid and hesitant, but she now believes in doing what is right.
It's somewhat like Dalinar's Oathbringer flashbacks, I think. You see Dalinar at his worst and his lowest points, and then you witness his transformation. Since Venli is a quieter character, her highs and lows are not as intense as Dalinar's, and I think that's why Dalinar's transformation is more appreciated among the fandom.