I know it violates your stated desire for a diaphragm pump, but I think the Marco gear pumps are the cat's meow. I think you'd be happy with a #3 or a #6.
They are simple, robust, quiet, and very compact with continuously variable speed so supply pressure is constant. We have had one for almost 5 years, and haven't needed to work on it yet. A spare parts kit of a shaft seal, motor brushes, and a few gears and they should go almost forever.
I personally would not go to a 55 PSI pump. That is just an invitation to waste water. We are very happy with a steady 25PSI.
Charleston, SC, USA