[Brightcolours]

Since I'm not using the 24-85 on FF, I don't have to worry about the outermost image field. Maybe (probably) it's going to suck there, but does it matter for me? It was pretty good on 8MP APS-C (just checked the old review here), and I have no complaints even now. Maybe I have an exceptional copy, but then again, my first copy was also very good and I only got rid of it because I gave it away for free to a good friend in need of a standard lens.

Speaking of native options, well, I guess I would've wanted to go native where possible if switching to mirrorless, especially since some ML lenses are unique and offer advantages over their SLR counterparts (say, this 14-35 vs. a 16-35). Otherwise I might have just as well retained my full EF kit and only used it, adapted, but then what's the point of even switching the bodies?

No, your copy is normal.

The issue with the 24-85mm is the border/corner performance, which you are cropping out.

You can see how much the 24-105mm f4-7.1 would trounce the old EF 24-85mm f3.5-4.5 by just looking at the MTFs.

The EF 24-85mm MTFs are on the bottom, and they are of the time that Canon published wide open and stopped down to f8 MTFs in one graph. The worst, lower black line pair corresponds to the 24-105mm black line pair,  same as with the worst lower blue line pair.

The old MTFs look a bit better than they should in comparison, things have changed a little bit with Canon MTFs over the decades too (they get closer to "1.0" than they would nowadays).

Of course, nothing wrong with picking the best lens if that is a native lens. But saying "I want to just go native" and also saying "With the bodies of different formats, you're getting even more possible combos." are two competing and incompatible viewpoints, that beg for a bit more nuanced approach.