Does the prayer become invalid if the worshipper decides to suffice with invoking blessings upon the Prophet (peace be upon him) in the Tashahhud, then makes a deliberate gesture indicating the unimportance of completing the Tashahhud, and then performs the Taslim?

What the questioner did by gesturing with her hand unnecessarily does not invalidate the prayer, but it is disliked by some scholars. And sending blessings upon the Prophet (peace be upon him) in the final Tashahhud is not a pillar of prayer according to some scholars. So, if the questioner is following these scholars, her prayer is valid and she does not need to repeat it.