The title says it all. Just wanting to see what you all have worked with and have experience with in this regard. 
Probably not what you expect as an answer, but if you’re looking for something to store and sync LONG notes that you don’t need to edit collaboratively, Joplin could be it.
There are options to self-host (WebDAV or its own server), and it’s just Markdown so real easy to backup, migrate etc.
Not an option for collaborative editing, and I’m not sure if you can publish if self-hosting.
That being said I’m also curious what other options are available!
You can self host an obsidian sync server iirc, or use file sync. Not quite the wsiywg UI of Apple notes but it’s reasonable.