If you don’t think it gets hot as hell then why do you suppose they left 4k60 off in the first place?
Well the R5II is a new camera body, it is expectable that not all possible video modes are implemented at the time of release.
By the way 4K60 is already implemented, 4K60 downsampled (= HQ) is not (yet) implemented.
As mentioned, I expect heat generation and therefore max. recording time somewhere between 8K30 and 8K60. Of course, the sensor has to capture 8K60, while downsampling to 4K60 is less computationally intensive than full processing of 8K60 (= 4x the data of 4K60), also writing to the card is only 1/4 for the same codec, consequently the card will most likely not overheat.
Note that with firmware 1.6.0 or later installed the R5 (Mark I) records 4K24 HQ longer than 8K24 (see //community.usa.canon.com/t5/EOS-DSLR-Mirrorless-Cameras/R5-firmware-1-6-Overheating-Fixed/m-p/380097):
Auto power off temp = Standard
4k HQ 23.98 warning @ 31m 02s, shutdown @ 40m 18s
8k IPB 23.98 warning @ 25m 50s, shutdown @ 33m 57s
Auto power off temp = High
4k HQ 23.98 ran for 3 hours, no warning or shutdown, gave up
8k IPB 23.98 ran for 4 hours plus, white warning after an hour or so, red warning intermittent, no shutdown