I made the mistake of defaulting to my life-long go-to brand, Sandisk. Pretty much every micro SD< SD, and back in the day compact flash, card I own is/has been SanDisk. Figured that'd be the route to go with the R5, and upon seeing the price tags, settled on the 128GB SanDisk CFE card.
About half a day later I stumbled across the Canon 'supported' card matrix and finally realized the smaller SanDisks have a lower write speed than the 512+. There was no way in hell I was getting my R5 in hand a day later and NOT be guaranteed solid 8K (despite having ZERO use for 8K....)
Placed a rush order with B&H immediately for the Sony 128 due to it being on the supported list and not the most pricey thing on said list. Of course, testing 8K on the Sony was successful (didn't even bother testing it on the SanDisk.)
I'm still keeping both though, since 99.999999999999999999% of my use will NOT be shooting 8k, and easily 80% of that will be stills. The speed that these cards offload to the computer is ridiculously fast so I definitely don't regret picking them up. I mean, they're literally NVME SSD drives on a PCI Express bus, shrunken into a camera.
Makes me wonder how I ever 'endured' transferring a day's shoot off of relatively speedy UHS-I SD cards.
All paired with the ProGrade UHS-II SD/CFExpress combo reader, which is also scary fast, and works well directly latched to both my Samsung Tab S6+, and iPadPro m1.