I read this book written by one of the psychologists who was doing early research into psychedelic therapy when I was a kid called "Thanatos To Eros, 35 Years of Psychedelic Exploration" and it was really interesting. It's still available on maps.org (Although they kind of muntzed the formatting) [1] but the takeaway is that besides MDMA there's a whole range of other substances with a variety of therapeutic merits to explore. MDMA may hit a sort of sweet-spot quality for more people to tolerate it without adverse reaction kind of sweet spot, however I think the substance Stolaroff was most interested in for enhancement of insight was actually very small doses of 2c-e (you can ctrl+f for it in the link), which I think is something we might end up seeing in more scientific investigations as interest grows.
[1] https://maps.org/2014/11/19/thanatos-to-eros-35-years-of-psy...