Is the aforementioned wording for praying upon the Prophet, may God’s prayers and peace be upon him, correct before the Tasleem?

The aforementioned form of sending blessings upon the Prophet, peace and blessings be upon him, is correct, and it is valid to send blessings upon him with it, both inside and outside of prayer. It is also valid to send blessings upon him with any form, though the most complete is to use the form mentioned in the question.