How much time did the classes take, in terms of upfront investment and any ongoing investment required?
I took 10 lessons, and I think I could probably have done it with 8. I still feel myself sagging now and again, and then I have to correct myself (and then I wonder why I sagged to begin with, because the correct posture is actually more comfortable). I have also bought a new chair ( https://www.ikea.com/nl/nl/p/kullaberg-bureaustoel-zwart-90325518/, only mine has a different color) and removed the backrest. I have found that chairs with a backrest and armrests tempt you to lean on them, and then you can end up sitting in a wrong posture for a long time without noticing. Without them, I soon notice and can correct myself.