One of the essential but most difficult jobs of a manager is to create psychological safety: the ability for people to raise what’s really on their mind, and to do so with the people who can act on that information.
I think you made the right call here. Telling your manager you were disappointed with their handling took courage, too. Now they can look into it further, starting with talking to the people you named.
Maybe they were just going along with you, “oh yeah, that guy’s a real jerk.” Maybe (and more likely) they didn’t raise it themselves because they’re don’t have the senior mindset you do. If your manager asks them directly, they’ll probably get a reasonably honest answer, although it’ll likely be softened a bit.
> It was also hard to get to a concrete, specific, actionable behavior change to request the difficult engineer to commit to.
Oh yeah, that’s the hell of these things. You end up playing whack-a-mole, too, because HR wants something more concrete than “don’t be an asshole,” and it turns out you can follow the letter of a PIP while finding plenty of other ways to be an asshole.