So I got to try out the new derailleur hanger alignment tool, super fast to use and next to no play at all. Cleaned out the threads with a tap to be on the safe side and was surprised to see my rear derailleur was out by around 8-9mm at the rim on two out of four points.
Really simple to tweak it back in, I know it is pretty leveraged but I was surprised how little force it took to bend even with the steel cross check dropouts. Shifting is markedly improved as a result!
https://flic.kr/p/2mhhwe6