Sadly, Syncthing is dropping Android support.

Another place to start: Privacy Guides has a history of tracking quite a variety of computer networking tools (browsers, data providers, Internet services, software, hardware, desktop and phone, even operating systems),

The EU has such, General Data Protection Regulation (GDPR), works reasonably well. Pretty good place to start.

Get a Proton account, you can then sign up for, or disable, various email notifications.