Time to give a little update on my chair situation, since I promised to do so.
After a lot of consideration, I've decided to return it. I really wanted the chair to work for me. In fact, I can say that it definitely DID help with my back pain, which became immediately apparent after getting back onto my old chair, because my back immediately started hurting again. However, the pain in the butt just didn't go away, like the manual claimed it would after a while. In fact, a little bit into the second week, I felt like it was actually getting worse. Considering the chair's high price, returning it was the reasonable decision.
Unfortunately, the shop demands a fee of 80€ for returning the chair, because while average people can assemble it, dismantling it is only possible in a factory, meaning it can't be returned via the same shipping method that it came with. While I'm not the one who has to pay for that money, I still feel slightly bad.
All of that being said, I haven't given up yet. As I mentioned, while the chair turned out to be a torture for my butt, it DID actually help with my back pain, so I know that I'm onto something with it. Therefore, I have decided to try out a different model of the same brand. This chair was called Aeris Swopper, but there's also a model called Aeris Swopper Air, which is the exact same chair, except with a seat that's a lot softer. I don't know if that has any disadvantages of its own, but some sources specifically recommend this chair to people for whom the regular model is too hard, so I still have some hope that it'll work out for me.
Feel free to visit my website/blog - it's updated rarely, but it looks pretty cool!