As-salāmu ʿalaykum wa raḥmatullāhi wa barakātuh, Is it permissible to recite Sūrat al-Kāfirūn and Sūrat al-Ikhlāṣ every time in the Sunnah prayers of Fajr and Maghrib, or would this be considered bidʿah? Jazākallāhu khayran.

Now you are following the Sunnah; the Messenger did it. How can something that the Messenger did—which we often say we follow—be a bid'ah? Of course not. InshaAllah, you can also recite something else from time to time, just like the Messenger did.