While I'm only interested in an open source solution, the YAML formatting they are using is almost exactly the style I was looking for. That would be a huge improvement to the current tooling available. I didn't see anything though, without signing up, that indicates that this would work well for mindmapping as well and implementing features like summaries:

https://xmind-help.github.io/en/summary.html

I build https://sourcespy.com to achieve the same goal - automated layout, drilldown but also ability to pull up the diagram contents from the code.

Just tried checking it out and the site appears to be down.