What is the best seat wedge angle?

It depends on how much L5-S1 curvature your body is capable of. If that joint has become stiff, you don't want to push it beyond what is comfortable. Look for a pelvic position that is comfortable and allows an easy stacking of your vertebrae. At first, this may not work very well. In this case, just practice for a few seconds / minutes, and then return to stretchsitting. Eventually, stacksitting will become very comfortable.

I am so glad that this approach is working for you. Yes, sadly, there is a lot of poor posture around. Do spread the word! People are always so happy to find something simple that works.