2005 buick rendezvous, low fuel pressure.

I’ve been having problems with poor gas mileage and lack of power even after i practically rebuilt the upper part of my engine, new injectors, fpr, plugs/wires, front 02 sensor, head/intake/exhaust gaskets, new maf sensor, cleaned up throttle body, new pcv valve, crank position sensor, radiator, thermostat, water pump, fuel filter, air filter and even ran fuel injector cleaner through multiple tanks. the fuel pump reads 40 psi on my cheap harbor freight fuel pressure tester, thought maybe it wasn’t working properly so i borrowed a nice one from a mechanic friend, it reads 38.3 psi at idle, and reaches 47.2 max driving down the highway. with the fuel pressure regulator’s vacuum line disconnected, the gauge reads 48.1. when i reconnect it drops back to 38.3. when the engine is turned off the gauge read 46.4 and dropped to 46.2 after 45 minutes of resting. i’m not sure if its a bad fuel pump or a bad fuel pressure regulator(which now has maybe about 1,000 miles total on it.) any help would be greatly appreciated. almost forgot, 2005 buick rendezvous cxl(i think), 3.4l 2wd 120,000 miles.