In Malaysia it is common practice after prayer for the Imam and the congregation to perform/recite the Istighfar, praises to Allah, the Fatiha, Ayah ul-Kursi, Takbir, Tahmid, etc. together. Finally, the Imam will read Dua' and the congregation will raise their hands and saying A'min. Is this Bidah considering that the practice is so widespread and regarded by many as proper?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The best guidance is the guidance of the Prophet Muhammad and his Companions and the worst are innovations. The practice mentioned in your question is not from the Prophet nor from his Companions. Therefore, it is an innovation.

If this practice would have been a good act, then the Prophet and his Companions, etc. would have performed them before us. The Prophet said: “Whoever introduces into this matter of ours (Islam) that which is not part of it; it will be rejected.” [Al-Bukhari and Muslim]

What is proved from the Prophet and his Companions is that they used to recite the supplication individually, not in congregation.

Allah knows best.