I have seen power steering pumps output low pressure at lower RPM so it is possible the pump is at fault. An easy test would be to fire it up at the dock and get her warmed up. Then see how it feels at idle. Increase RPM to 1500 (in nuetral) and see how it feels then. If no better or only slightly better, increase to 2000-2500 RPM and see how it feels. If it is getting better at higher RPM than most likely it is the pump or the increased pump pressuse is eough to overcome whatever else is causing the issue
So the time I had seen a pump with low pressure was on an old Diesel Grumman truck (Chevy chassis). It as brought in because the brakes were hard and it was difficult to stop. It had power brakes but since a diesel does not produce vacuum like a gas engine, it used power steering pump pressure for the power assist. After testing it out for a while we noticed that the brakes were only hard at idle. If we drove the truck, popped it in nuetral and kept RPM at 1500, she would stop on a dime.
So we replaced the PS pump and all was fixed.
Now imagine the driver of the trucks face when he came to get it. He asked what it turned out to be and when we said "it was the powere steering pump", he looked at us like we were crazy
After a more detailed explanation he started to get it and after he drove it and saw it was truly fixed, he never doubted us again