Thanks for your observations. I fully agree the insulation of especially the freezer is the main problem. As Oliver suggested in previous postings, changing the insulation with a better one really makes a big difference. But he also said it's a hell of a project. In the mean time we are using circulation pumps like cookies.
Fact is that Flojet claims to supply you a pump for continuous duty but it's not. Even at 80% duty cycle the thing drops dead after less then a year. That should not happen. As said by others, brushed motors are simply not suitable for this task.
So the idea of replacing the pump-motor itself is quite good but you will need to carefully select what motor to replace it with.
I've looked at the sea-water circuit and I'm thinking about a way to clean it, but that is not so simple. I think the easiest is to pump some descaling fluid though the circuit and let is soak for a while. On the other hand the continuous flow of water makes keeps growth of whatever wants to settle in the circuit also minimal.
It's a pity that Flojet does not sell a brushless variant of the pump. That would make the problem much better to cope with.