ECC tests are rare but X670E ASRock’s been done at least twice: Wendell and ApplesOfEpicness. As @quilt mentioned there’s more general Level1Techs threads for this: AM5 ECC and Ryzen 7000.
ECC support’s mostly an AGESA thing, suggesting QVL or ASUS versus ASRock isn’t significant here. So far as I know only Nemix does kits, Kingston, Apacer, and Axiom sell singles. Maybe there’s others.