I think you are right to question water. The Three Waters thing got needlessly politicised. In the future, we'll see more shared infrastructure and more "shared blueprints" for how to do things as we go forward. I don't agree with the current central government's austerity (they recommended shuttering all regional councils) but I do think that there is a lot of overlap in services. It's pretty amazing how much autonomy the councils have, but one problem with that autonomy is that many of them reinvent the wheel.
Organisations, like LGNZ which you linked to, are already working to streamline things and provide economies of scale to local councils.
The problem with debt is that it is not used like LGNZ described. It is used to shift the burden down to the next batch of elected officials and to our children. This is done without enough public consultation. My view is that there isn't enough long-term accountability, and we need some sort of meaningful cap on rates rises; with exceptions for emergencies of course. Anything above and beyond that budget should have to go for a referendum: e.g. "Let's take on debt to build a sports field!" should be voted on by constituents.
That would require increased engagement in local government which has been in serious decline as documented by numerous inquiries over the last 5 to 10 years.
It's a wicked problem, but it's not insurmountable if regular people activate and push for the right things.
Have you read any Karl Popper?
Yes, and: Elected members seem to treat them the same; which is why I keep conflating them in my posts.
Popper wrote "The Open Society and Its Enemies (1945)" while at University of Canterbury:
https://en.wikipedia.org/wiki/The_Open_Society_and_Its_Enemies
This book came to mind when I wrote "... if regular people activate and push for the right things."