What is the ruling on the imam's commitment to reciting specific verses and surahs repeatedly, and what are the implications of that for praying behind him?

The first and best is for the imam to be the one among the people who has the most memorization and the best recitation. There is no harm in regularly reciting specific surahs in prayer, as long as one does not believe in their sunnah status or their obligation. Therefore, the prayer behind the aforementioned imam is valid, even if he repeats specific surahs.