Is it obligatory to maintain the iftirash posture while sitting between the two prostrations in prayer, even if it causes pain, or is it permissible to sit in any other manner to achieve tranquility?

Iftirash (sitting on the left foot with the right foot upright) in prayer is a Sunnah for which one is rewarded if performed, and not punished if omitted. If it is difficult for someone, they may sit in any position that brings them comfort.