Partly for my own use but largely for learning purposes, I developed a multi-platform note-taking app using Flutter and Dart. It's a bare-bones mobile-first app that stores notes as *.md files to support SyncThing-based synchronization. The UI is inspired by Android apps like Omni Notes, Material Notes, Noteless, etc., which however are either not multi-platform or not aimed at SyncThing-based synchronization. Obsidian's mobile app, in my opinion, is not as mobile-friendly as these apps. Joplin is the closest I could find but which I did not like.
It's tested only on Android 10 and Windows 11. Bring done with Flutter, it should work on iPhone, Mac and Linux too but would need building, testing and fixing various issues found.
Had I known this would take me 3.5 weeks (dedicated time) and 6100 lines of code (including comments), I would not have done it. Ideal would have been just a week.
Currently closed source.