2001 Hyundai Elantra Hard Start in Morning with P0507 DTC - Fuel Pressure Issue?

I recently experienced a hard start issue with my 2001 Hyundai Elantra, where the engine won't crank past 7-8 attempts before starting. After replacing the fuel pump module, fuel injectors, spark plugs, wires, crankshaft and camshaft sensors, the car still struggles to start in the morning—sometimes requiring five key cycles to start. Last week, my wife drove it 35 miles to work and reported that she could only maintain a speed of around 40 mph; the vehicle lacked power when pressing the gas pedal fully. Upon returning home, I scanned the system and found DTC P0507. What should I do next? Any guidance would be greatly appreciated. The symptoms—hard morning starts and lack of acceleration—are concerning. Given that the car started immediately after spraying starting fluid into the intake, fuel pressure is likely a contributing factor. A P050-7 code typically indicates an issue with engine idle control or fuel system performance. Possible causes include: - Vacuum leaks - Leaking air intake downstream of the throttle body - EGR valve vacuum leak - Faulty PCV valve - Damaged or dirty throttle body - Failed EVAP system - Malfunctioning IAC (idle air control) or faulty IAC circuit I have already replaced the fuel pump twice and installed a complete fuel pump module, but the issue persists. I now suspect that the fuel pressure is insufficient despite these replacements. Next steps include checking fuel pressure and inspecting for vacuum leaks or throttle body issues.