I considered the same dilemma myself a few months ago and it boiled down to this:
My need for pro-video features = less, I am a purist when it comes to shooting moving images - no peaking, no zebra, no elaborate extras, no clutter on screen, I have very basic audio needs, far more need for a fast operating nimble camera that can switch quickly between the highest possible video specs and nicest possible ergonomics for stills, so for me the EOS R5 was a better deal than R5C, far less money used, it behaves in a more nimble way for smaller scale creative endeavour and filmmaking, with better AF and is a superior stills camera, the overheating aspect is much improved vs the launch, and the hardware was never really limited very much by raw thermals anyway - it was all silly firmware trickery.
So I recently went R5, but the mount just doesn't do it for me.
With the Nikon Z8 I can use many more adapters, many more of my existing lenses, far more comes off my shelf and is happy on the Z8, especially for stills.
So it really boils down to lenses and whether you'd be happy with a normal R5, before considering the Z8 as a step up from that.