Use fuel return line for extra gas tank vent?

Noticed my fuel pressure fluctuates a lot (up to 1.5 psi) when driving with a full tank. When it's half full it fluctuates a little, and when it's 1/4 full or less it's almost rock steady at 3.9 psi.

Seem like the tank's not venting enough. Cleaned and tested the flow valve a while back, think it's OK but not sure. Not using a vented gas cap. Was able to blow through the line from the flow valve to the tank or wherever it goes, but it might be partly blocked with corrosion or something.

Not using a return line, maybe it would work as a tank vent, unless there's something else I'm not seeing. Don't want to cause a gas smell.

According to ZTherapy video, fuel pressure affects the A/F mix.