Thats provided a great understand of what's going on.Loss of fuel pressure and either sucking in the tank walls or sending unit seal. Along with that comes either loss of performance, parts damage or both. If you car is anyhting other than NA, then the result is generally you driving over parts ( )
The cap is made to relieve pressure from the fuel changing volume due to temperature changes - that is a very slo process and the cap is adequate for that.
The sending unit should have a port made to be a vent - but it's very popyular to plug it. Are you still using the factory sending unit?
This is easily diagnosed with a fuel pressure gauge in the car (electric of course).
I've completed an LS Swap but not up to speed with fuel systems. I've had the inline pump replaced with an in tank Walbro 255, at the same time I got a new tank and factory FI 3 outlet sender. The mechanic hooked it all up.
Needless to say I got half way through a tank of fuel before the pump noise got super loud. I cracked the fuel cap and fuel poured out.
I spoke with the mechanic and the 3rd line out of the sender had been internally vented to the filler neck. Nothing to the outside atmosphere other than the cap. This was rectified today but the pump is busted. Still whining and bit of loss of power.