Intersting point. I am curious about how the pressure is measured.

You commented on 3 psi -through- the filter. Does that mean into
rather than out of? Can you tell about the into and out of pressures?

Reflectint on your system: You have mentioned three main components:

Pump: plumbed in and hard to move. Seems clear of debris and has free
impeller. Water flow into the pump appears free. The system was
working with this arrangement. Supposed to handle up to 2" junk. If
junk through is larger than the outlet and is fairly solid, it could
clog the pump at the outlet and restrict the flow.

Pipe: 5-6 ft long. Not yet clear if it is plugged in some way. If it
is smaller than what the pump can send on, i.e. less than the exit of
the pump, it could be plugged by the pump. I suspect the pump,
however, has a smaller outlet than 2". Perhaps the pipe could be
snaked or have an hose pushed into it to flush it. If it has turns
and if something came out of the pump that could not clear the turn,
you could have a problem that you would not find.

Filter: Easiest to clean or to inspect. I gather you can check out
the pressure in some way and perhaps make a visual inspection. Would
a clog or jam show up? Could you recognize it?

Interesting and painful problem. I suspect lots of us would like to
jump into your pond!

Have I missed any components?

Jim