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@z428eu
As you've mentioned Nextcloud there's always WebDAV.
https://github.com/stefandanzl/webdav
Personally I use Syncthing, but you may run into issues, as Android FS supports less characters than a modern ext.
@z428eu
For Android you can try the Syncthing Fork, as it has more options. I'm driving it daily with minor issues only (ST has it's limitations).
https://f-droid.org/packages/com.github.catfriend1.syncthingfork
@z428eu I've tried AppFlowy, it's not for me, but it worked.
I've landed on Joplin for note taking with the data sitting on my Nextcloud.
Because Joplin, as you know, doesn't do relational notes...
Logseq + rclone here, and does the job perfectly (without Nextcloud, because it is a major PITA).
I'm having a good time with Obsidian + Koofr + FolderSync for Android synchronization. It's a little fiddly to set up because Koofr wants you to generate individual app credentials rather than give your user credentials to client apps.
That's probably sensible from an access control perspective, but might make a few users do a double take.
If you don't tell FolderSync to watch for changes, you may find you have sync problems, which is annoying. I tend to manually sync before editing.