Is saying "Astaghfirullah wa atubu ilayh" (I seek forgiveness from Allah and repent to Him) after the final Tashahhud and the Ibrahimiyah prayer, and before the Tasleem in congregational prayer, considered an addition that invalidates the prayer?

It is sunnah (definition) to seek forgiveness after the final tashahhud (definition) in prayer. The reported formulas for this are: "O Allah, I have wronged myself greatly, and indeed, no one forgives sins except You. So grant me forgiveness from You, and have mercy upon me. Indeed, You are the Forgiving, the Merciful," and "O Allah, forgive me for what I have put forth and what I have delayed, and what I have concealed and what I have declared, and what I have exceeded in, and what You know better than I. You are the Advancer and You are the Delayer. There is no god but You." It is preferable to learn these formulas to supplicate with them, and the prayer is not harmed by what has been mentioned.