I'm a fan of active sitting. In Germany we have "Aktion Gesunder Rücken e.V." (working group healthy back) and they publish a list of "Active office chairs" (Aktiv-Bürostühle) with their seal of approval at
https://agr-ev.de/de/ratgeber-produkte/produkte/2837-aktiv-b...
These chairs aren't for everyone, i encourage you to try them before you buy!
The ones i know personally are
- Aeris 3Dee
Super nice chair, very ergonomic, quite pricey, starting at around 1000€
- Dauphin Stilo with 3d-balance system
Bought this chair for my adult son who is super happy with it and uses it lots. Costs around 550€. I may buy one for myself, too.
- Haider Bioswing
I bought a used chair with this mechanical principle and it's pretty good but not as good as the other two, at least for this (quite old) chair. A new one would probably be more ergonomic