"etc" is doing a lot of work there; what in your view would constitute success here - something that would allow us to say "we did it, we now have the technology to freely change our sex at will"?

It depends on how you define sex. Hormones cover most expressions from chromosomes already. Skull reconstruction could be better? Maybe some way to really rework skeletons and adjust height and hips or shoulders? But that's more on the cosmetic end.

Or if you want at will, it would be good to have some way to radically speed up changes. You're still looking at 3-8 years for some things.