High Fuel Trims and Low Fuel Pump Voltage Question

The vehicle is a 1997 Chevrolet Silverado K1500 with the 5.7L Vortec. It had a no start condition that was initially diagnosed as a bad fuel pump. Replacing the fuel pump did get it running again, however, it still starts slow at times and runs a bit rough. Plugging it in I find that the fuel trims are running high (~+9-13 both banks at idle, up to +20 or more under load) with bank 1 being slightly worse than bank 2 but it is an old truck. I put on a fuel pressure gauge and found the pressure is running consistently about 10psi lower than normal. That is, 50-53psi at idle, jumping up a few PSI and then dropping down to around 47-48psi before coming back when hitting the throttle. The next thing I have looked at hard was fuel pump voltage and current. I have <.5V voltage drop at the harness going into the tank, and no problems with the ground wire going to the frame. I even checked to make sure there is low resistance between the frame and the battery. However, I still only have 5.3A current being drawn by the pump when measuring at the test lead in the engine compartment. I have come to the conclusion that it has to be one of these problems: wiring issue inside the tank, the new fuel pump is defective, or a problem with the fuel pressure regulator. I, of course, don't want to blindly start buying more parts, what else can I do to diagnose the cause of the low fuel pressure before I start diving into dropping the tank (again) and pulling the plenum?

how is the fuel filter? i finally seen a restricted fuel filter cause low pressure. only seen it once and it was on a 1999 suburban

A clogged filter would make the amperage go up.....

If this is still the spider injection with a dedicated return line you can block it and see what happens to the fuel pressure.

With an amp clamp you can calculate pump speed.

Disconnect the connecter at the pump, measure the pump's resistance and compare to specs.

I did the pinch test and there was an immediate change in the sound of the fuel pump and spike in amps going to the fuel pump. I think that eliminates any chance of it being a wiring problem or defective fuel pump. The regulator must be letting fuel pass at a lower than normal pressure. I feel kinda silly for not having done that test before posting this question...but still learning here:) Last question just for better understanding is why would the regulator act that way? Just a spring getting weak? Or maybe some piece of debris stuck in it that got introduced during a fuel filter change?