I hope some of this might help.
It seems to me that the EOS R5 is very sensitive to temperature. As the temperature increases, so does the noise (graininess). The biggest difference between the EOS R5 and your 5D Mark IV is the IBIS and the higher resolution. IBIS means that it is more difficult to cool the sensor chip. Higher resolution on the same size sensor means that each photo site on the sensor collects fewer photons with the same exposure so that the probability of unexpected luminance or color is higher. Higher temperatures will also increase noise in the 5D Mark IV.
I try to avoid using the big screen and use only the viewfinder to reduce power usage since power use turns into heat. In the power settings, I set the screen and viewfinder to turn off quickly.
Also, it seems to me that enabling any of auto lighting optimizer, highlight tone priority, peripheral illumination correction, or portrait lighting in the camera menus makes it more likely that there will be noise. I enable these later in the Canon DPP software if that is desirable.
In Canon DPP, one may see the noise reduction that the camera would have chosen. The camera will reduce the amount of digital lens optimizer as the noise increases.
One strategy for dealing with the noise would be to apply noise reduction, then downsample, then apply unsharp mask. For noise reduction, Canon DPP works for me and other software also does noise reduction. A median filter would work for noise reduction before downsampling. I expect that one would not need to downsample to the resolution of the 5D Mark IV to get noise similar to the 5D Mark IV at the same ISO. I might downsample a noisy image to 75% (I use graphicsmagick for this, but other software also has good algorithms). Unsharp mask should have a radius larger than any remaining noise to avoid sharpening the noise.
exiftool -CameraTemperature *.CR3
will report the sensor temperature.
