Wa'alaykumsalam Wa Rahmatullah
Saying Sadaqallah Al Azeem is not part of the Sunnah, there are no reports that this was said after the recitation of the Prophet Muhammad peace be upon him or any of his companions.
Saying it after each and every recitation of the Quran may lead to an Innovation.
Sheikh Ibn Baz said that it is OK to say it once in a while BUT not everytime as it is not from the Sunnah,
Allah knows best.