You can select a local folder in Obsidian for Android and sync the folder with Syncthing. You can even revoke network permissions for Obsidian and it all works completely offline (Flatpak override: –unshare=network / GrapheneOS: don’t allow the network permission).
This is my current setup, even though Obsidian is not FOSS. I like that it stores standard Markdown files in a traditional filesystem hierarchy, instead of what Joplin does with using Markdown files as a database. This means that with Obsidian I can use any text editor or any other Markdown app to access and edit my notes, whereas with Joplin I would have to export them first to standard Markdown and then potentially rename and reorganise all the files and their attachments.