[john70t]

Offhand, I would tend to think "over-pressurization" will never cause full throttle WOT stumbling.

Low FP pressure problems usually only occur under high load..
So forget about "too much" line pressure. (i.e. that relief valve)

Put a fuel pressure gauge on the rail schrader valve, same as a bike pump, and measure it directly!
(And forget about what the lying computer says.)
It will be par for the course I'm just guessing.

As a stupid mechanic from the internet, I suspect the throttle body is gunked up.

There is a throttle bypass mixture circuit.
The Mass Air Flow/Manifold Absolute Pressure/combo of the two might be off.
(I honestly don't know what the modern pcars cars use)
Some "carb cleaners" will damage the delicate MAF wire. Don;t spray on that wire. Be warned. Use the approved ones.

Another possibility might be hard mechanical problems.
Valves are sticking or worn.
EGR is stuck open.
A vacuum leak from a hose. Use propane and see if idle speed changes.

Another possibility might be low system voltage at idle may confuse the computer.
Car computers don't function below 9V.

Start from the basics:
Battery. Alternator charging amps at idle and 3,000 rpms.
Those are known good.
Move onto the next set.
It's a huge list. One step at a time. It's all practice until the Olympics.