Is it permissible for me to choose whatever I wish of dhikr after the prayer, or must I adhere to what was authentically reported from the Messenger of Allah, peace and blessings be upon him?

The remembrances (adhkar) reported from the Prophet, peace and blessings be upon him, after prayer are supererogatory (mustahabb), and performing them at their designated time is better than at other times. There is no harm in performing other authentic remembrances, provided that one does not commit to them and consider them a Sunnah. For continuous adherence to a specific remembrance at this particular time requires evidence; otherwise, there is a fear that it might become an added innovation (bid'ah). Goodness lies in following the guidance of the Prophet, peace and blessings be upon him, as he said: "Whoever performs an action which is not in accordance with our matter, it will be rejected."