Is the prayer valid if the opening supplication (Du'a al-Istiftah) is recited after the first Takbir (Takbirat al-Ihram) and not before it?

Your prayer is valid. The mentioned supplication is one of the opening supplications (Adhkar al-Istiftah) that are recited after the Takbirat al-Ihram (the opening Allahu Akbar), not before it, as has been reported in multiple authentic narrations from the Prophet, peace and blessings be upon him, and as stated by the scholars.