How do I get more shower flow volume?
I would suspect you have a two fold problem. 
1: Your tankless water heaters ability to heat and deliver water - GP/m.
2: Your city pressure (indicated by your outdoor faucet) - not sure if you tested that with the faucet removed or still attached;  Just because you have a certain amount of pressure from the city does not mean you have the same flow rate. Your flow rate might be different as in GP/m's supplied at X pressure.

Item 2 is what I would examine - check with your municipality to see if there is a flow rate restriction of some kind (what is their spec for flow rate at what pressure), or perhaps your house water supply valve is not open all the way.