See page 32 of the User Manual for details on compatible cards. I don't see UHS-II listed, so you'll need to stick with UHS-I cards of which I believe they max out at V30 (30 MBps sustained writes).
Page 150 then outlines the various bitrates for various settings. The Highest bitrate I see is for capturing UHD footage with the XF-AVC codec: 160 Mbps which is 20 MBps. So a V30 card will be capable and have some additional headroom.
For cards, I would stick with only very trusted brands to include SanDisk. And then purchase them from only reputable dealers (e.g. avoid Amazon as counterfeit cards exist there). Also, be sure to purchase full-sized SD cards only and not micro-sized that would require adapters.
Finally, on the various V30 rated cards to look at, the most prominent number printed on them will be the theoretical maximum read speed. Higher values there can help when its time to copy footage from them to your computer. Ultimately though, it's best to look at the V-rating since that describes the sustained write capability. Too bad card manufacturers over the years chose to print the theoretical reads in the largest font.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.