You're right the Bucky stuff is a significant part of the movie. The weakest part. And you reminded me that even so, Tony and Steve is the real relationship that matters in the movie. Theirs is the friendship that is (kind of but not really, because it's Marvel) ended. Theirs is the relationship that defines the conflict. And you're right, the end of the movie is about Tony vs. Steve. Bucky is a cause, but the focus is on two Avengers with their hands at each other's throat.
Further Stark is the one with the arc in the movie. He goes from egotist loner to feeling guilty over global instability, to being the one who tries to rein Cap in, to becoming disillusioned in his friendship with Cap, the government he trusted, and arguably even the Avengers themselves. He drives most of the conflict of the movie, even if he ends up being an antagonist.
I won't dispute that it is called Captain America: Civil War. But the reason it is so good, and the reason it made a billion dollars, is because it is really an Avengers movie that co-stars RDJ. And I wouldn't be surprised if most moviegoers remember it as simply "Civil War" or an Avengers movie. That admittedly I can't prove, but when James Gunn is irked that THR is saying he is an "Avengers director," I think it's clear that audiences and even trades aren't sweating the specifics as these lines blur more and more.
It's why I really appreciate Black Panther being so standalone. Yes, there are a few allusions to Civil War in it, but other than the post-credits scene, it is a completely autonomous and original movie. And, I would argue, it is better for it.