DeGoogle Yourself

Interfacing markdown:

• Joplin (pretty popular, haven't used)
• Standard Notes (same as above)
• Markor (feature-packed, a bit outdated)
• BeautyXT (maybe a bit too minimal almost, but gets the job done if you just want text based editing)

Interfacing orgmode:

• Orgzly (beautiful simple app that uses a superior syntax when compared to Markdown due to its built-in agenda and to-do structure)

Syncing flat files (might not be compatible with all of the above)

• Their own hosted server if available (easy but not private, some provide E2EE)
• Syncthing (easy and private)
• WebDav or an SFTP server (hard if self-hosted or otherwise easy through some providers)
• Termux + a Git server (pain)

I use Markor on android, plus a self-hosted instance of codeserver for editing online. Synced via syncthing. Sometimes i use Obsidian with the same synced folder.

If you have a nextcloud instance, and you don't need Keep's collaboration features, Carnet is FOSS and is a decent solution

I use zettel notes and sync with syncthing. On pc I use neovim with marksman and zk.nvim

Joplin + Syncthing