I do have a setup with Android, Syncthing and Tasker. Whenever Tasker sees a specific text file when checking periodically, it generates a notification for me.
This gives me the possibility to notify me on all different stuff just by creating a text file whose content is the notification message.
You can use that for Android-local notifications as well as for notifications from any device with access to the Syncthing share.
HTH
Okay, it seems I’m not even halfway there to run NixOS on my own …