Here is a sneak peek of the 8K recording options for the upcoming Canon EOS R5.
As you can see, you can shoot either 8K DCI or 8K ultra, which will be slightly different resolutions. The resolutions of the two modes will likely be 8192×4320 and 7680×4320.
Codecs for 8K recordings appear to be ALL-I, IPB, and obviously RAW.
8K 30p and 4K 120p will not have a crop.
There will be no recording 8K video to both cards simultaneously.
Outside of environmental factors, there will be no overheating issues when recording 8K for more than 15 minutes.