EteSync Server VS DecSync CC

Are you / have you considered leveraging postgres' row level security for this (row filtering)? Feels like a natural fit for ensuring the right subset of data gets to the right users https://www.postgresql.org/docs/current/ddl-rowsecurity.html

E2E encryption would also be nice to have, it could be worth having a look at https://www.etesync.com/ for inspiration

A few days ago I saw some guys in this community saying that they uses https://www.etesync.com/ in order to sync contacts between Proton + Thunderbird (Desktop) + Mobile. I haven't used it yet, but I saved the link.

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EteSync - Secure Data Sync (version 2.4.0): Secure, private and end-to-end encrypted calendar, contacts and tasks sync

The programs will allow me to sync both my Google and my Etesyc Contacts and Calendars. Etesync also needs Etesync-Dav installed on the desktop but that is well explained on Github.

There is https://www.etesync.com

Afaik they use databases, and synching databases is bound to corrupt them. Generally you need a program to be aware that something can be accessing and even writing it's files at any time, as that requires special care (see e.g. for calendars and contacts: https://github.com/39aldo39/DecSyncCC). I'd look for an extension or something to export/import that data to files, which you then can use together with syncthing for a semi-manual process.

I have experienced this issue: https://github.com/39aldo39/DecSyncCC/issues/83

There are android versions of Syncthing and decsync on Fdroid or Google Play. IMO the only downside is, there is only a native plugin for Evolution. There is a plugin for radicale, which is a Caldav/Carddav server. So you can locally run radicale with decsync plugin, sync decsync via Syncrhing and sync Thunderbird, etc via radicale. This is still what you are looking for, but it is a little complex setup imo...

I have Tasks.org + DecSyncCC for android, with Radicale Server + Thunderbird for the desktop.

In alternative you can use DecSync CC, serverless sync with multiple devices using Syncthing (or other sync) https://github.com/39aldo39/DecSyncCC

DecSync CC (version 2.0.1): Sync contacts, calendars and tasks without a server using DecSync

I'm not sure I understand correctly, but take a look at this: https://github.com/39aldo39/DecSyncCC

Have you tried DecsyncCC? It creates a CardDAV and CalDAV account on your phone to synchronize your contacts, calendar, and tasks to a local directory.