I don't think Tazawa has the raw overpowering stuff to be a closer. He always had good command from an early age, and he has decent secondary pitches. I just don't think he has dominating closer stuff. As for Uehara, I have never understood how he can throw 89 mph fastballs by hitters, but he does. I like him as a late inning arm, but I don't trust his stuff enough to have him close on a regular basis.
Miller has overpowering stuff, and when he has his stuff together, he can be untouchable. His problem is having consistent command of his stuff. Of the internal options, he would be the first trial at closer. The second option would be Alex Wilson. I have liked what I have seen from him in his appearances.