What should one do if, during prayer, they realize they did not achieve tranquility (طمأنينة) in a pillar or action until after moving on from it? And what should one do if they are praying with a congregation? Is it permissible for someone who realizes this during the action itself to correct their posture?

Tranquility during bowing (rukūʿ) is a pillar of prayer according to the majority of scholars. The worshipper must bow low enough to be able to place the palms of his hands on his knees. This is the minimum sufficient amount. As for the most complete form, it is to straighten his back and neck. The Prophet, peace and blessings be upon him, when he bowed, would place his hands firmly on his knees, then straighten his back, neither raising nor lowering his head excessively. If the worshipper performs the necessary amount but not the most complete, he should adjust his posture. He is not required to repeat the glorification (tasbīḥ) after the adjustment, and his prayer is valid.