Is repeating istighfar (seeking forgiveness) 20 times after every prayer using the mentioned formula—then supplicating with praise to Allah, then asking for one's need, then sending blessings upon the Prophet—considered an innovation or a recommended act? And is supplication after prayer a time when prayers are answered?

Remembrance of Allah (dhikr Allah) is a great act of worship that a Muslim should constantly perform and increase. It is permissible to use non-traditional dhikr and supplications occasionally after prayer, without making it a habit.

However, persistently reciting specific dhikr that are not authentically reported after the obligatory prayer is an additive innovation (bid'ah idafiyyah) that should be abandoned, based on the saying of the Prophet, peace and blessings be upon him: "Whoever introduces into this matter of ours that which is not from it, it will be rejected."

Supplication after the obligatory prayer is one of the occasions when supplications are answered.