fuel pressure dropping over time

The return orifice in production filters is most likely there to smooth out the pulsating pressure at lower rpms that mech pumps generate.
The example in post #64 of the orifice suddenly becoming large[r] because of solder detaching & causing fuel starvation is different to the OPs set up. The OP has a bypass regulator where the carb takes the fuel it wants before the excess fuel is returned to the tank. In the example in post 64, the carb was getting fuel after the fuel was returned to the tank, which starved the engine of fuel; the carb was the last cab on the rank, not the first as in the OPs case. There is no such thing as a return line that is too big, only too small [ within reason ]. It is a small return line that causes the flow to back up & interfere with steady pressure. Which is why the Mallory 4309 instructions recommend a larger return line with increased length.