At my place, I have a small DC grid with a varying voltage between 20 and 50v. I use it for quite a few things, including supplying power to most of my portable devices and everything else that is always on, except for my USB devices (of which I have many).

At the moment, I am powering them all through a cheap LM2596 module each, but I was thinking about what would be the most efficient way to power them all.

Efficient in this case means: idle power draw (when no USB devices are connected) should be really low, but at full power draw (when all connected devices use 3A each) converting losses should be low. Obviously, most of the time, something in between would be used, probably more at the lower end.

Would I be better off using many small buck converters or only one large one? (Are buck converters even the right way to go?)

Paraphrasing the question: On a wide range of input voltages and output currents, what would be the best way to ensure maximum efficiency?