HiHamsterLuna

We can safety rule out the sensor heat as being the main culprit if people are able to record 4KHQ externally for hours on end.
It is either something going on with the CFExpress cards getting to hot or some kind of timer that is started when they are inserted.
The fact that the camera power cycles when the memory or battery doors open may be a hint that that may be the case.
Please keep testing @Andrew Reid and @mechanicalEYE very helpful for people. 

R300 looks great! The C300III sensor is really amazing. RF mount also opens the possibility to use a speedbooster to get full frame on a ~$6000 small body with internal ND, pro codecs and probably external RAW, way better than the C300III for some people who doesn't needs all the features from the C line.