How is your posture? You may go further by strengthening your back, hips, neck, shoulders. If every chair is uncomfortable it could mean the issue is more in your own physical state. You could invest some money in a physiotherapist to check you out.
I've had more impact through planking and thoracic spine stretches than any chair could ever have (your own case will be different, of course)
The goal isn't to be able to sit still for hours on end, that's your body telling you to get up and move. But you should still be able to sit for generally without feeling uncomfortable.
Consulting a physio has been well worth it, just be prepared to invest some time each day in some targetted exercise as that's where the gains are.
Now I no longer have to worry about what chair I have -- well as long as it isnt an Iron Maiden :)