While seeing a bit of a dip from the R5 at ISO 100 on mechanical shutter may be a bit sour, there's a few good news stories for measurebaiting in there.
The Electronic shutter on the R5ii is notably better than on the R5 at lower ISOs, and better than the Z8 at every ISO from 100 and onward, while the mechanical shutter is better at ISO 100 than the Z8 is at its base ISO 64. For anyone actually leaning on the speed of the R5ii, that's a pretty good story - less rolling shutter, more dynamic range at high-speed, and definitely very comparable (to really any other full frame camera) dynamic range at peak with mechanical shutter. With that said,
Bill Claff also noted that while there is noise reduction at all ISOs on the R5ii, there is less noise reduction on low ISO in the R5ii than in the R5. I'm not sure if that will improve shadow detail, but it may counter some of the concerns attached to noise reduction being baked in to spoof DR at low ISO.