The current Canon firmwares don’t implement pacing when the buffer is full. The firmware waits for the buffer to clear a bit, shoots 5 shots at full fps, waits form the buffer to clear a bit, shoots 5 shots, etc.
Nikon does it like you describe: write out each shot after it is taken, so you get a nice, even fps that is limited by your CFe card. The Z9 and Z8 are also able to write to CFe at 900-ish MByte/s, the R5 barely reaches 500MByte/s. Both are below the 1500Mbyte/s practical CFe2 limit.
Bring on CFe4!