The part where you said that it's clear that Tagiru won't ever surpass Taiki is EXACTLY what I don't like about his character, actually.
While I do agree that Taiki is severely underused (JUST STAY THE MAIN CHARACTER, GODDAMN!), I disagree that he's the least bit dull. It's Tagiru to me that is dull, because he's just so much more-of-the-same. Interchange Tagiru with all the hot-blooded heroes this metaseries seems to love putting out (with Takato and Taiki being the only breaths of fresh air) and you won't get much of a change. It's the huge differences between Tagiru and Taiki that is exactly what still makes Taiki infinitely the better character... His story continues to build on top of itself. He still fills the mold as leader, still think of all the strategies, but now he's got experience. He's a veteran. He's a hero. He actually stirs a response from the rivals. He assumes duty for what it is, while for Tagiru it's just a game still. I would be more okay with Tagiru if he even showed signs that he is worthy of surpassing Taiki, but just.. absolutely not.