I think the R and RP and probably a lot of the Canon mirror cameras all will get just as hot as the R5 when doing only photos. It's just that no one paid attention to it till now with the R5/R6 release. Someone mentioned that the R manual also stated the same heating issues with photos only.
IBIS is new now for Canon, so maybe that is causing more heat in these models?
But that can be turned off.
Unlikely, evidence points towards an inadequate thermal interface between the hottest parts (likely microprocessor) and the heatsink (body).
Camera overheats but the body is cold = there is a layer between body and internal components that act as an insulator. This explains also why the bodies take so long to recover after overheating.