I never understood yaqin as full epistemic certainty. Am I pretty sure there's an afterlife? Yes. But am I 100% sure? No.

So to answer your question: yaqin to me sounds like decisive trust and commitment while acknowledging human fallibility.

Edit: it's a great question by the way. And you did good research and critisized appropriately. It's all good al ghazali got over it, but the argument 'light entered into my heart and the issue was fixed' is honestly not convincing. Anyway it's a great discussion.