Many people use Disroot to store and synchronize their contacts and calendar events using the CardDAV and CalDAV protocols, which are provided as part of the Nextcloud service. Its much better, than sharing this data with Internet companies like Google or Microsoft, but still not perfect: this data is not end-to-end encrypted, therefore Disroot admins (or someone who can compromise or hack the disroot server) can access the data.
Recently I found a project that allows to synchronize contacts and events with end-to-end encryption, so even if the server got hacked or improperly accessed, the user data will be safe because only the client application can decrypt it, not the server.
The project called EteSync and is free and open-source software. EteSync Android App is a fork of DAVx Android App (ex DAVdroid) and is available on F-Droid. The server is also open source and can be self-hosted.
I think it would be great if Disroot can setup and host its own EteSync server so the users can sync their contacts and calendars with end-to-end encryption.
Note: I am not connected in any way with EteSync team. If you know anything that says against it, let me know. The only drawbacks I found so far: there was no cryptography and security audit and the app is quite new and not time-tested.