I have personally used both SanDisk and Angelbird cards. My new favorite is Angelbird.
One important thing to note is that if capturing 8K, you'll need to do so to the CFexpress card slot (many of the codecs involving 8K requires the higher bandwidth of CFexpress. Even a V90-rated SD card will not be fast enough).
Though for your secondary slot, highly recommend a V90-rated SDXC UHS-II card.
The most important card specification to look at is the sustained write speed. One reason I ended up primarily moving to Angelbird is that they show that value as the most prominant info on their cards (i.e. largest font/typeface). SanDisk will instead put in largest type, the theoretical maximum read speed which isn't useful at all when speaking to capturing photos/video.
Always stick will full-sized cards. Never use cards that require adapters.
Back to read speeds... sometimes very difficult to find is what the sustained read speeds are. Higher read speeds can be good in potentially allowing faster copies from those cards to your computer. I say potentially in that it will all depend upon your computer storage write speed and how you're connecting things to read the media cards.
You may want to also invest in a backup solution. While the EOS R5 does have dual card slots which you can set up to ensure photos are saved to both, any videos that can only be recorded to the CFexpress card will not have any backups. Portable drives that either have built-in card readers, or able to be connected to dedicated card readers, would allow you to make copies.
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Ricky
Camera: EOS 5D IV, EF 50mm f/1.2L, EF 135mm f/2L
Lighting: Profoto Lights & Modifiers
