Is it necessary to do tawbah or if someone has too much yakin on Rehmat of Allah swt that Allah swt will forgive him his mistake and he does not says astagfar then will Allah swt forgive or Allah swt had told us to perform astaghfar ?

And in astagfar if one has too much yakin on Allah's mercy that as he do niyat of astagfar he believes he would have been forgiven and feels not needed of saying astagfar then is he true ?

Knowing Allah is Merciful does not permit us to just go about doing sins and not even saying we are sorry for them and seeking his pardon. We would not accept this from another human being. Imagine if we are merciful but someone keeps trampling our rights and treating us bad then they never apologized. We would not accept this from them and be upset with them. We must do tauba and ask Allah’s forgiveness and not feel arrogance that we deserve Allah’s mercy.