Hi Julian:
As regards the 75-300... Everything I have read indicates that this is arguably the worst lens that Canon have produced for image sharpness and resolution. I had one that I inherited with a kit about 15 years ago (and I realize there have been several iterations of the lens since then), so I would be interested in your views as a user of the MKIII version.
I have personally always preferred to have the image as close to right in the camera as opposed to correcting in PP, so I sold the 75-300 and opted for the Canon EF 70-300mm f/4-5.6 IS USM, of which there are two variants: the MkI issued in 2005 and the MkII in 2016. If you want to see my review of these two units see my post:
Canon Non-L 70-300 Canon Lenses