Probably not the answer you want to hear but the best office chair is no chair. I only have a standing desk at home and my back/legs are so much better after 3 weeks of WFH than they have ever been.
I'm almost 2m tall so always had back and chair problems so I took the opportunity to switch my desk to a standing one for 4 months with a college who went on a project abroad. It was hell from the first to the last day. Not only was standing problematic, it distracted me so much I got angry and used every opportunity to get away from it and to sit and talk to other colleges in their offices (been told about it at some point because I didn't even realise it).
I'd say if you're not OK with it after 4 months, you probably won't be after 4 years.
I joined a swimming club again. It helped.