You've been calling it a "filter" and all the time I've been thinking that is odd terminology to use. What you want to install there is a "sea-water strainer". That could be part of your problem ... but it doesn't explain why flow is weak when you eliminate the "filter". You've been after this problem for a long time, it seems, without any satisfaction. This should be a relatively easy problem to resolve. All it takes is a functioning pump and clear hoses and fittings. It should be easy to identify any blockage or failing part. Trying to resolve this issue while you are not in the water does complicate matters. Sucking water out of a bucket may or may not affect how it works, but it should be a relatively good approximation of the real deal.
I'm not familiar with this system (as David explained it) at all. I have no idea what is your purpose for showing that hose that is obviously cut or damaged somehow. It appears that there may be a thin flexible hose within? Is that hose that you are holding part of a re-circulation system or something else? There is a gate valve and it is closed. If it is part of a re-circulating system, why would you have to manually close and open that valve? Or should it be open? The re-circulating system certainly does complicate matters.
A failed impeller sometimes has no obvious defects. I've replaced several failed impellers in my smaller diesel that looked no different than a brand-new impeller. Yet the failed impellers, as good as they looked, were definitely not passing water, where a new impeller would immediately work as intended. If you haven't replaced your impeller, or your pump for that matter, you may be searching for a solution when the obvious answer is staring you in the face. I apologize if I am missing a few of the steps you have taken but it seems like it has been a long road so far and perhaps it is time for a mechanic's fresh eyes to resolve this issue.