What is the expiation for an oath that was not fulfilled after swearing by Allah to wait for a specific person, knowing that the relationship later developed into marriage, and the non-fulfillment of the oath was discovered after the marriage, along with repentance to Allah?

If by your oath you intended to refrain from accepting a marriage proposal or speaking with a person for the purpose of marriage, then you have done what you swore to abandon. You are required to offer an expiation for breaking an oath: feeding ten poor people or clothing them. If you cannot find them, then fasting for three days.