Been detailing this in the Canon Rumors discord. But I've found that if there are no cards in the camera, you get 4 hours of external recording before "overheating". I use quotes here because after overheating and shutting down, a quick 5 min rest allowed me to shoot that last 20min segment in 4KHQ, with still no warnings - and I expect it to last at least another hour or two. I tested this in the video with a dummy battery, but afterwards tested with a new Canon LP6H, and the battery died before any warnings.
This leads me to believe there is some type of temperature limitation going on with the cards. Either the cards are triggering an aggressive overheating timer, or the cards themselves are what's overheating to the point of turning off. This makes sense, as the only thing that is hot is the card. During these 4 hours, the camera is nearly cool to the touch, with maybe some warmth on the back panel.
I will be doing some outdoor tests to see if ambient temps play a role. But the original "only 45 min / 75 with overheat control on" does not seem to be the case without SD/CFEx cards installed.