A friend had a similar car with a similar problem. After doing all you have done and more, we concluded the torque converter is the problem.
Other years and models with basically the same engine have a torque converter that lets the engine rev to about 2400 rpm. At 1500 rpm not much is happening in the engine.
Don't mess with the pump unless the chain is very stretched; and if that is the case, replace the chain before attepting to adjust the pump.

Could you explain that logic?

Sure, the pump timing isn't that far off if the chain is not stretched, hence it doesn't need to messed with.

If chain stretch was the only factor affecting injection timing, that theory might have some validity.