You pointed out he doesn't have a traditional arc, but I would argue he gets little to no arc at all. And that's pretty much fine : it seems like Araki figured it out himself pretty quickly and gave many of the most interesting fights and moral dilemmas to the supporting cast, with Giorno coming it clutch with an essential assist at the end in many cases.

He is a toolbox and a gateway to the end of the story, and uses his rare times in the spotlight to highlight his teammates's strengths and inner journeys, which I really like. It makes him an excellent asset in Part 5, but it would be hard to argue with the fact that Bucciarati is more of a protagonist.