I have used Prograde Cobalt series cards in a Canon R3 for the last two years. While a slightly different model I would assume that they would work flawlessly in the R5 as well. If the Cobalt series is too costly you could also consider the Gold series although the buffer depth might suffer a bit because the Gold series does not have as fast as a minimum write speed as the Cobalt series.
I would buy the largest card than you can afford and if long-burst at 20 fps is not your thing a Gold Series card would probably suffice.
Lastly, Prograde also announced CF Express Type B 4.0 cards which are twice as fast as the older 2.0 cards, but to my knowledge no current cameras can utilize the faster bus speed. With that said, with the 4.0 cards can upload data faster to the computer. However, the upload speed could be limited by other factors such as the connection to the computer.