Are my prayers valid if I recite the Tashahhud from Ibn Ashir's متن (text), which includes "and I bear witness that what Muhammad brought is true, and that Paradise is true, and that Hellfire is true, and that the Sirat (bridge) is true, and that the Hour is coming without a doubt, and that Allah will resurrect those in the graves," and may I continue to do so in my future prayers?

Various forms of the Tashahhud have been narrated. The additions found in Imam Ibn Abi Zayd al-Qayrawani’s Risala, namely "And I bear witness that what Muhammad brought is truth, and that Paradise is truth, and that Hell is truth, ﴿and that the Hour is coming, there is no doubt about it, and that Allah will resurrect those in the graves﴾" and "and have mercy on Muhammad and the family of Muhammad," have not been reported in any authentic hadith. Rather, the first of these was narrated with a very weak chain of transmission, and scholars have criticized it for not being authentically reported. The correct approach is to omit it and restrict oneself to what has been authentically reported. Then, the worshipper may supplicate with whatever he wishes after the Ibrahimiyya prayer. Your previous prayers are valid because these additions do not invalidate them.