Only-Antonis posted a video to YouTube with R6 4k/25p rolling shutter and 4k/50p overheating measurements here:
https://www.youtube.com/watch?v=9CnRhEUGpcQ
It looks to me like 33ms rolling shutter in the 4k/25p footage.
In 4k/50p the sensor overheats at just under 29 minutes- not including a short pause at 19 minutes where he re-formats the 32GB card.
Rolling shutter test at 7:45 to 8:10
Overheat test at 15:00 to 16:45