Zen5 appears to officially support up to DDR5 5600, but unfortunately all of the ASRock Rack or Supermicro boards I looked at only supported DDR5 5200.
I may wait for new Zen5 boards, or maybe take a gamble on something like the Asus ProArt, where I saw comments online indicating that ECC is (unofficially?) supported.
Looking forward to Ryzen 9000 ECC benchmarks.