I have complete my first prototype and push code to GitHub - wao/joplin: Joplin - an open source note taking and to-do application with synchronization capabilities for Windows, macOS, Linux, Android and iOS. Forum: https://discourse.joplinapp.org/ branch android_sync. I also upload prebuilt apk to:
Release Release for android background sync at 0103 · wao/joplin · GitHub.
You can try it at your own risk or you can build from source by yourself. It works great for me. I think it is enough for my daily use.