I'd only consider the Canon high MP body if it had dynamic range near the GFX system, but may not be possible on a FF sensor.
Everything you wrote up to this one sentence is no longer relevant. All KB-Sensors could not reach the gain of a medium format sensor. The pixels have a greater diameter and the pixelspace is geater. All of them are related to the performance values like S/N-Ratio and Sensivity.
I would like to have a 60MP resolution on a ff format sensor in a body like the R5. Wenni compare the 5Ds with the R5 pictures like doing macro photos, the 5ds pictures looks more naturaly that the R5. I own both and some other canons. The R5 hase some advantages. The sensivity is higher, the noise is quite less and the EVF is an advantage for me if i am do macros. I can see the motiv outdoor even the sun is bright. The OVF digital systems are designed and optimized for the automatik focus use-case. Manual focus operation is not good supported. The all misses a traditional focus screen. for bodies like 5DM4, 5Ds and later, this is not available to buy and mount as an add-on.
And yes, all the lenses needs to be in good quality too, if the sensor resolutes more than 50MP., to so the advantage on the picture at the developing process.
Canon had decide to be an expensive system. I yould not buy Canon again, as long they not license their RF Mount to third-party lense buildes like Sigma.
Actually they had done this but only for the APS-C Systems. I assume, they don't want to build better lenses for APS-C and let Sigma and Tamron doing this.
The next System will be a Sony A7R5 or newer. Actual, my R5 and the R are the two bodies i use in most cases. I have all lenses that i require - EF-Mount mit Converter of course.