Diagnosing hard start, stalling, no idle on 1999 3.4l porsche carrera 911 996 - checking fuel system is OK

So, continuing on the saga of the 996 that barely turns over, trying to figure out if a fuel issue is causing it, as that's kinda what it sounds like to me.

Made a fused lead to jump the fuel relay pins 87-30 to manually activate the fuel pump, still hard start.

Then a thought - it looked like I had half a tank according to the fuel gauge, but the odometer read 290km or so, which is not what I can normally get on a half tank! Added 20L from a jerrycan, still same problem - no not a faulty fuel gauge then (could be empty but reading half full)

Next, to check actual fuel output.

I removed the return fuel line from the 3.4L - need a 17mm spanner. This is located under the coolant expansion tank, left side of engine.

Used some spare heater hose and a hose clamp to connect to the threaded end.

Then, used fused cable again to jump pins 30-87 on the fuel pump relay. Factory specs say there should be 850ml output in 30 seconds

Had a spare container which I'd previously marked 850ml - and guess what, exactly 850ml in 30 seconds. If there was less than this, I would test output after the fuel filter, and then again at the output of the fuel pump.

So, it appears the fuel system is OK - good pump, good filter, good fuel pressure regulator. Relay should be OK as well, since I jumped it and had the same poor idle symptoms.

Feels like I'm running out of cheap/easy things to test now!