Line Expansion Submersible pump

Put the pump in a 5 gallon bucket with two bricks in it; something to keep it off the floor of even it. Improvise some kind of intake screen if necessary.

Install a 'T' fitting on the output, valved, so that you can bleed off the desired amount of over pressure in the line directly back into the pond.

It's probably clogging where you have it choked down and possibly not making the pump happy from lack of cooling water flow.

My family had a pool that would be covered over the winter. We would use a simple garden hose to siphon the water off. The siphon would be started by putting one end in the pool and filling the hose up through the spigot. When it was disconnected it would start siphoning immediately.

The end of the hose was put in a beach play bucket with a brick in it so it would always be the lowest point and not get air in it.

We'd have to clear leaves and detritus occasionally from the hose end, but usually the leaves were decomposed enough that they'd flow through the hose.

Sometimes we would use the pool vacuum hose, but since it was so big (1 1/2" OD) it was hard to get a siphon started.