If the CFexpress is the problem, why the CFexpress card does not overheat when shooting in 4K 30p non HQ mode? I shot for hours in this mode to both CF and SD cards without an issue. Both HQ and non HQ modes got the same bit rate as per the manual.
Besides, both the CFexpress and SD cards return to room temperature in a matter of minutes. The camera, however, doesn't seem to be affected; it doesn't care even if I inserted a new, unused card. It won't work.
I believe that this theory is misleading.
The HQ modes sample from a higher resolution, but output at 4K - that downsampling requires Digic processing.
Non HQ modes - no sampling, less data (vs 60FPS or higher modes). Hence your 4k30p is ok - you're taxing Digic less and the I/O less.
Essentially when we go to the higher processing - be that FPS, Resolution (8k), Downsampling, some combinations are pushing parts (multiple parts) collectively to go over whatever thermal limit has been defined and you get the problem.
We expect Digic to have a temp sensor (most processors of some shape or form do), not sure about the (image) sensor - probably does, then there's CFE Cards (Delkin does, assume others do also). And then there's the PCIe bus which CFE uses. So we maybe have 3 or 4 temp sensors, and the higher processing could trigger any of those into hitting their thermal limit and you get the warning....
How the menu / screen / switch on operation fairs in all this - no clue - best guess is that is a bug and can be fixed. The rest of the system, tweaked sure to give a bit more headroom. Then other modes e.g. Raw Lite, will have less i/o, and hopefully less processing - that may be an option for some. I'd like to see them do 4K Raw and 120fps in HD as others have suggested as at least it will give options....