Sounds like crud, can you disconnect the fuel line from the tank blow air back thru just the line . Reconnect to tank see if the flow speeds up not a fix but that might define the problem. Cannot see, think of a way just the fuel line would be the bad actor, unless it is rubber and is collapsing inside.

Full tank not fun, reading that tank link I see this.
• Ball check vent furnished with tanks requiring non vented caps

Might be something to find and check, however "removed cap and not a very strong stream," still think it will be crud.