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I’m quitting my PhD. The decision has been made. The reasons boil down to diminished interest in the research subject and disappointment with the university’s managerial practices (nothing is done on time or well). A while ago I informed my supervisor about these concerns and gave him the heads up that I may be leaving after a period of self reflection. He found my concerns justified and made reasonable adjustments, what was within his power, but it finally rained down on me that no matter what I won’t pull forward the PhD.

We have an amiable professional relation and I’m sure he may be interested in knowing what went wrong and when. I also want to diminish the blow as I suspect he had put lots of hope on my research, not to mention that I was his first PhD student and my departure will be a hindrance to his tenure. He also has a bit of an ego which I'd rather keep appeased (This is in Europe, in one of the countries with more hierarchy-based academic systems.)

In hindsight, what concrete information would an experienced academician expect to get from a PhD who is leaving?

In general, I feel the two items you list already (over-)fulfill what justification you strictly speaking owe an advisor on your way out. You lost interest in your research subject, and you are disappointed with the university's managerial practices. I am not sure there is all that much more to say with regards to the first item, and it's not particularly actionable for the advisor anyway.

For the second item, I am sure he would appreciate some concrete examples of situations that you feel where inadequately handled. At the very least, this gives him ammunition for future discussions around improving managerial processes ("we already lost je_b because of situations X, Y, and Z" is a much easier to make argument towards change than an abstract "we need to improve some stuff").

The only areas where I would tread lightly is problems where he is himself the culprit, or could have reasonably made a difference but didn't. Personally, I would use the common advice from Workplace.SE here on exit interviews - don't use them as a chance to blow off some steam. The chance that you will enact positive change is low, but the probability that you burn some bridges is very high.

This is culturally dependent, of course, but almost all of what you say about him is positive. Therefore it might be a good idea to share it. If at some future time you decide to return to academia, perhaps in a different field or university, it would be useful to have left all bridges standing, with good feelings all around. If he is sorry to see you go and wishes you the best in all things, you have set a foundation for your future. You may need it or not.

But, depending on country and field, he may not expect anything. In that case, a positive statement, especially if written, can be especially helpful to your future.

It is also possible that if he agrees with you on the impediments in place by the university itself, that he may have, in your statement, some ammunition to try to change them.

Had your experience with the advisor been mostly negative, on the other hand, I'd recommend you say nothing. It is a bad idea to leave enemies behind you. Some other folks here leave because of advisor conflicts and even malfeasance. Sometimes silence is the better policy than honesty.