I'm still really hung up on the "between the 1DX Mark III and EOS R5," I think we may very well see the R3 coming in around $4500-5500 instead of approaching the cost of the 1DX Mark III.
Jordan from DPReview TV noted that he was felt like one way they may differentiate it from the 1DX mark III is if it has a cheaper 12 FPS mechanical shutter mechanism like the R5, versus the expensive 20 FPS 1D shutter mechanism. That could make sense, if Canon feels confident enough in the stacked-sensor 30 FPS readout speed to mostly replace the mechanical shutter, they could save some cost on parts. Realistically, this camera doesn't need a mirror, a fancy mechanical shutter, or a huge crystal in the eyepiece, so it could very well end up being between the two cameras in cost.