Yes, the Canon EOS R6 Mark II is compatible with the Sony SF-G Tough 128GB.
Sony SF-G Tough 128GB is V90 class. Meets the floor for: 4K30 (30 MB/s floor); 4K60 (60 MB/s floor). (Peak write 299 MB/s is not evidence of sustained performance; the V90 class rating above decides this.)
“Canon EOS R6 Mark II Compatible Cards: SD/SDHC/SDXC with UHS-II and UHS-I supported; for movies use a card with ample performance (fast enough writing and reading speeds) per Canon online manual.”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)