As a supervisor, what feedback would you expect from a PhD who quits?
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.