Recommendations for someone 6'3", 90kg with lower back pain and tight hip flexors? Mirra 2 isn't doing it for me

So I bought the Mirra 2 a while ago and have been determined to make it work for me but I think I've just been refusing to admit it doesn't. I get hip pain after sitting for not very long, and it doesn't seem to help my back, I've tried adjusting in various ways but it doesn't seem to help. Reading other threads, I don't think this is uncommon for this chair, so I'm wondering if other people have found good alternatives?