Personally, I've used Joplin [0] some months ago, and Im using QOwnNotes [1] now. Both are open source, cross platform, and can synchronize with Nextcloud. QOwnNotes may feel a bit faster since its written in Qt/C++ (Joplin is an Electron app).
However, I still find hard to write tables in Markdown manually.
Notes are stored in local db yet notebooks in the db can be mapped on file folders (e.g. folder on DropBox containing html files - each note is a single HTML file).
The note db file can be protected/encrypted by OS means.
Windows/Mac/Linux.
Currently using this Chrome extension: https://chrome.google.com/webstore/detail/marcador/nneoamjib...
Started as just adding bookmarks to your Dropbox, but now it lets you add plain text notes.
And it only has access to its own app directory--not your whole Dropbox!
Personally I'm having a good time with Tiddlywiki with a markdown plugin and nice context-of-backlinks setup (inspired by roamresearch.com )