Hi,
I would suggest removing the pressure relief valve, then see if oil flows out when the engine is turned over
Anti sumping valves are the source of all evil
If the feed side off the pump is working ok then I would remove the sump plate and use a pressure oil can to inject oil into the pickup pipe
If you can enlist a helper, remove the return pipe at the rear of the crankcase and turn the engine over while injecting oil into the pickup
oil should come out the return connection
Use the oil can to check that the return pipe to the tank is clear
It is not unknown for the pipes to collapse internally
It has happened that the pump gears have failed internally
John