Yes, the Canon EOS R5 Mark II works with the Lexar Professional 2000x 256GB SD — with conditions (see below).
Lexar 2000x writes at 260 MB/s. Canon R5 Mark II SD slot supports 4K30 at lower bitrates -- 260 MB/s meets that SD mode floor. 4K60+ requires CFexpress Type B.
Conditions for compatibility
4K30 via SD slot supported. 4K60 and above requires CFexpress Type B in primary slot.
“Canon EOS R5 Mark II Compatible Cards: SD slot accepts UHS-II and UHS-I; for movie recording use a card with ample performance for the movie recording quality per Canon online manual.”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)