Seems like a process of elimination here, start with cables, then hanger, then mech, then bike, then rider! etc etc. Trouble is it's a second hand bike so I'm just not entirely sure what to expect in terms of wear and tear. Thankfully the jockey wheels aren't anywhere ninja stars. I was just wondering if the shifting weirdness on big ring and the 3rd/4th biggest cogs might point to something specific. They aren't worn down by any means. Hanger is suspect but I just can't gauge it by eye. Should probably start with a new inner and a hanger then look to replace the mech if I get nowhere with that.
Did notice the B screw seems to have no impact on the positioning of the chain to cog gap?