To what extent is it valid to attribute a specific supplication to 70,000 angels who carry it for whoever recites it after Fajr prayer, and is part of this supplication "O Allah, bestow upon me of Your bounty"?

No supplication with the mentioned wording has been reported. However, a supplication from the Prophet, peace and blessings be upon him, is said when going out for prayer, and it states that whoever says it, "Allah will turn His Face towards him, and seventy thousand angels will seek forgiveness for him." The angels' seeking forgiveness in this supplication is not exclusive to Fajr prayer. Scholars have differed regarding the chain of narration of this hadith, between classifying it as good (hasan) and weak (da'if).