It sounds like you've got enough bearing wear that the oil pump can't keep up with the leakdown rate. Drop the oil pan and inspect the bearings, measure the crankshaft for size and roundness, and check the clearance with Plastigauge. In 50-something years of fixing cars, I've only seen two oil pump failures that resulted in low/no oil pressure, and both of them had a piece of debris in the relief valve.
Jerry

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