Is it obligatory to re-pronounce the Two Testimonies for someone who committed showing off (riyaa'), which is known to be a minor shirk, and is this act considered a major shirk, knowing that repentance has occurred through remorse, seeking forgiveness, and a firm resolve not to return to it?

Your repentance for feigning prayer actions to deceive others is sufficient by desisting, regretting, and resolving not to return to it. You are not required to re-utter the Two Testimonies (Shahadatayn) because you are judged to be a Muslim, and certainty is not removed by doubt. What you mentioned, that an abundance of minor Shirk (polytheism) can lead to major Shirk, is correct.