Is it permissible to say: "My God, You are my aim, and Your رضا is my quest"?

This phrase is permissible, but it is not a reported (ma'thur) remembrance (dhikr). It is better for a Muslim to be keen on the prophetic remembrances (adhkar) that are reported, because the Prophet, may Allah bless him and grant him peace, was the most knowledgeable of people about Allah and the best of them in praising Him. He was given comprehensive words (jawami' al-kalim), so his remembrances were the best of remembrances and his guidance was the best of guidance. The Companions were keen to learn the remembrances from him; Abu Bakr Al-Siddiq, may Allah be pleased with him, asked him to teach him a supplication (du'a), just as the Companions asked him about how to send prayers upon him.