Notesnook is a free (as in speech) & open-source note-taking app focused on user privacy & ease of use. To ensure zero knowledge principles, Notesnook encrypts everything on your device using XChaCha20-Poly1305 & Argon2.
I’ve not tried the app myself, but what does stand out for me is the free plan does have unlimited notes, and I must say that the Android UI looks very good. Too many note-taking apps (e.g. Obsidian) have a difficult to use mobile interface.
A possible downside of the free plan is that there are no attachments or images, and tags are limited to only 5. That said, I suppose about US$2 per month is not too band for the Pro version, and that also works out to about half of Obsidian Notes’ Pro version.
It supports Markdown and basic rich text formatting. A further plus is block based linking (like Logseq does) where you can link directly to headings or paragraphs in remote links.
Exports are limited to text only for the free version versus the Pro version with PDF, HTML, and Markdown.
But the open source code is really only getting you the client app. So this is not the same as say Joplin where you also get the code to self-host the sync server.
See https://www.androidpolice.com/tried-encrypted-all-in-one-productivity-app-blew-my-mind