And your source for that statement is where?without any water being under pressure, which for a 500 l tank will be considerably more than 85 l. While a 500 l tank will not give you 500 l it will usually give you about 70% under normal domestic pressure - about 350 l
I can provide tables for pressure tanks used for smoothing out pump cycling. So far you have made claims without any source to back them up
The pressure in any pressure tank is supplied by (in rural Thailand) by the water pump so its usually going to be between 1.9 and for a more expensive pump 2.8 bar, only if you go to double the price will it be greater. This will provide between .16(.7bar differential) and a maximum of .36 (2bar differential) of the tank volume at any reasonable pressure.
and an example of "a constant pressure pump which will give no flow" is?An in-line pump will give far better flow, without power, than any centrifugal
pump of the type you have pictured or described, particularly any constant pressure pump some of which will give no flow at all. This is (or should be) crucial when choosing any pump for a gravity-fed system where the power supply is unreliable.
What is a booster/in-line pump?
A type of centrifugal pump used to increase the pressure of liquid that is already flowing from one place to another
I had thought of a pressure tank and dismissed it as being an inefficient expensive option. Roof tanks work, are easy to add to if needed, supply the total amount of water stored, and are cost effective. As to the pump, it is you who have become fixated on the one I mentioned. If you bother to actually read what is written I never said that it was the pump I am going to install. I haven't got to the stage of deciding on pumps yet.
They are the "pitfalls" you imagine, if they exsist I will post about them.What I do think is worth commenting on, though, in case anyone reading this takes your system as a known one which will solve their problems, is the pitfalls inherent in it.