IMO the simple solution to this problem is showing Taln's life before the oath pact and how flawed he was before becoming an icon for perseverance. This would align with the themes of broken people becoming more than anyone thought they could ever be, especially themselves. You say that that level of determination can't be cultivated, but I think it can. I think Kaladin post 4th ideal is starting to do something that seemed impossible for him in the past and that's accepting the death of those he couldn't save and finding meaning in the moments they have. Who's to say Taln couldn't have experienced a life where he lands of an understanding of the world that makes him unbreakable in this specific situation? Especially since the Heralds are supposed to be the blue prints for 5th Ideal Radiants. The comparison to the other Heralds can be problematic tho. I think an elegant solution to this would be to frame the breaking of the other Heralds as more of a difference in philosophy than a lack of merit. The other Heralds wonder if what they are suffering for is even moral. Are humans even entitled to Roshar? Are they on the wrong side of ethics? For Talns philosophy these questions are irrelevant and he will serve. For the others they act as the cracks that eventually break them time and time again. This doesn't place Taln over the other Heralds as much and puts into question Taln's unrelenting devotion. I think Taln can be handled well, hope he is done justice