What is the ruling regarding prayers I performed with minor errors in the Tashahhud, and by pronouncing "ṣalli" without a kasra?

Adding "ya" (يا) to "peace be upon you, O Prophet" (السلام عليك أيها النبي) in the Tashahhud does not invalidate the prayer, although it is better to avoid it. As for saying "I bear witness that Muhammad is the Messenger of Allah" (أشهد أن محمدًا رسول الله), it is permissible and mentioned in some narrations, but what is preferable is what was stated in the hadith of Ibn Mas'ud: "I bear witness that Muhammad is His servant and His Messenger" (أشهد أن محمدًا عبده ورسوله). The word "salli" (صلِّ) should be pronounced with a shaddah and a kasra under the lam, without a ya. There is no need to repeat prayers that were performed incorrectly before knowing the correct way.