Herman Miller Sayl review

User is deciding between a used Herman Miller Sayl and a new Sihoo Doro C300 for 10-hour workdays with mixed sitting/standing. They are 5'8" and 165 lbs, cannot test in person, and are concerned about whether the Sayl's backrest is supportive enough for their height and whether the Sihoo is genuine quality or just influencer hype.