Obsidian? Saves everything in Markdown. It is offline but you can sync things with syncthing. Has a great app on mobile too. I run syncthing on my phone too and sync everything between my devices.
This.
I sync my (markdown) notes between Linux, Windows and Android devices, edit on either device , or even manually manipulate the text files, and all is well.
I use markor on android (open source) and syncthing to keep my readable MD files synched with my silverbullet self hosted, which is also nice for online editing at the same time.
Beware that Joplin MD files are not standard and cannot be easily read standalone.
Its a common misunderstanding. Joplin files are .MD, and somewhere it says its markdown, because indeed Joplin uses markdown even inside the files.
But if you do open those files with any other markdown editor, you find out they are not plain markdown. The text itself is mixed up with other stuff (tags and such) in a non standard way. You notes are not lost and can be recovered, just not by straight open up in another markdown editor.
I don't use Trillium, but I've enjoyed Logseq. I use it with nextcloud to back up. It is not browser friendly as far as I know when self hosted. It's primarily just markdown.