I am not sure who that level thing on the EOS R will be implemented but I am sure ISO could be reached from the super screen or what ever they are calling it. Canon has taken a very different approach to mirrorless than Nikon, I was a bit surprised that they did not offer IBIS in stead on in lens stabilization.
Regarding mirrorless lenses it appears that the difference in the setback between mirrorless lenses and DSLR cameras with the mirror makes a difference in the size of the port needed for wide lenses. Example Nauticam recommends the 250mm port for best results with the Canon 11-24 F/4 and the Nikon 14-24 F/2.8. For the Sony 12-24 F/4 the 230mm port is recommended for best results. While Sony 16-35 F/4 & 2.8 can be used with ports as small as 180mm. So apples to apples on full frame Sony at 12mm behind 230mm dome should yield the same corner sharpness as Canon at 11mm behind the 250mm given the same F/stop.
The prevailing wisdom is that Nikon D850 and D500 have the fastest and most accurate AF of any of the DSLR's and that Sony has the best for any of the mirrorless cameras. I Shoot the 12-24mm in AF-C most of the time and I do not own a focus gear for the lens, in fact the only focus gear I own is for the 90mm macro and I would be hard pressed to say the last time I have used it. Like all full frame cameras I try to shoot at F/13 or higher unless I am looking for shallow DOF. This is one tradeoff for shooting FF. Keep in mind that I am not a video guy so my observations are based on stills. The high end Sony lenses are excellent which has been proven by tests by DXO and others. Also regarding the larger lens mounts selected by Canon, Nikon and Panasonic I understand the concept for faster lenses but the smaller Sony mount seems to be fine for any lens I would use and you can find a bunch of lenses in the F/2 to F/1.2 range that are offering excellent results on Sony FF.