As you say, an R6 or R6II (or even the R8) plus an RF 24-105/4L is a great combination. When you consider the comparison to the crop 17-55/2.8, the 24-105/4 is effectively wider (24mm vs. 27mm), longer (105mm vs. 88mm), faster (f/4 vs. f/4.5 for DoF when matching framing), and with the FF sensor you gain 1.3-stops of noise or shutter speed that more than offsets the one stop lost in lens aperture.
I think that pretty much seals it for me, thank you for spelling it out so clearly!
The only thing I will be missing will be reach, but it'll be interesting to see how much I actually miss it.
R6 II --- RF 14-35mm f/4L IS --- RF 24-105mm f/4L IS --- RF 100-400mm F5.6-8 IS
70D --- EF-S 10-22mm f/3.5-4.5 --- EF-S 17-55mm f/2.8 IS --- EF 70-200mm f/4L IS --- EF 85mm f/1.8