Must the dhikr be recited immediately after the obligatory prayer, or is it permissible to pray the supererogatory Sunnah prayers that follow, and then recite the dhikr?

The fundamental principle regarding the prescribed remembrances (adhkar) after prayers is that they should be recited immediately after the obligatory (fardh) prayer, not after the subsequent supererogatory (Sunnah) prayer. This is indicated by the noble prophetic traditions, as "al-mu'aqqibat" are words recited after the prayer. This does not negate the reward for someone who delays the remembrances until after the Sunnah prayer; rather, they are still rewarded. However, the preferable approach is to adhere to the apparent Sunnah to attain the full reward. Furthermore, performing the remembrance and supplication before the regular supererogatory prayer fulfills the Sunnah of separating the obligatory prayer from the supererogatory one.