Help Me, I am being dumb

I had an issue similar before and was caused by a loose jockey wheel, took me ages to find it as I was over thinking/complicating things when trying to diagnose, go back to the beginning and start with the basics first would be my advice
 
Your run of outer cable from the last stop to the rear mech looks maybe a bit short. Not going to allow the mech to move as it should.
 
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