Is there a way to invoke blessings upon the Prophet (peace be upon him) that would result in a reward multiplied by the number of Allah's creation or similar, and is there a specific wording for that?

There is no formula in the Sunnah that indicates a multiplication of the reward for praying upon the Prophet by the number of God's creation. However, the principle of multiplication is established: whoever invokes blessings upon him once, Allah blesses him ten times. The meaning of "Allah blesses him" is that Allah bestows His mercy upon him and praises him.

There is no objection to formulas of prayer that are not transmitted [from the Prophet], such as "as often as those who remember Him remember Him and as often as those who are heedless neglect His remembrance." However, it is better to limit oneself to the transmitted formulas, such as the Ibrahimiyyah prayer, which is the best of forms, because the Prophet, peace and blessings be upon him, taught it to his companions after they asked him.