I can understand your frustration. I do believe you are winning though. Even if it was the filter, that is more of a clarity issue, not a chlorine demand or trying to get cc's down. If it was strictly the filter causing the problem, you wouldn't be fighting chlorine loss like you are. Just a dirtier pool. Understand? I just ran a full test on my pool before this post. Although my chlorine is down again, I have zero cc's and the pool is clear as a bell. It has been since I pulled the cover off this spring. Therefore, I still think I have some demand from the citric yet. That could be part of your problem as well. There are others on this site that have experienced what you are experiencing, believe me. Some have had bacteria over winter convert to ammonia. Others have converted from using Bacquacil. It has also been a long struggle for some & both of those situations take enormous amounts of chlorine & a long time in some cases to clear out. One thing I do know, Chlorine is the MOST effective and the MOST cost effective route to fix it.
Just keep adding chlorine, & try to have some more patience with it. It will get there. It will!
Keeping us updated, & asking questions is fine.
One quick question, in one of your pics you stated at 6:46 am you had 17.5 FC. I had been under the impression you shocked to 19. That is 1.5 loss. Am I missing something? Tonite again, if you can, add to 19. Wait at least an hour, preferably just before bed recheck but don't add anything, & then recheck early in the am before the sun hits. I'd like to see an actual, reliable OCLT, not that you aren't doing it already.
Also, water needs to be circulating well when you test. If you can, run the pump all night doing the test.