[runjake]

Somebody’s going to com in here and mention Obsidian Notes, so it might as well be me: https://obsidian.md

It supports collapsible sections just with the Markdown syntax but it’s also highly-extensible in any way you wish. I think Bear and other editors support Markdown collapsible a as well.

[bmallerd]

Seconding obsidian. Throw the files into Dropbox and voila - synced notes.