Xandikos VS DecSync CC

Looks like you looking for CalDAV server, xandikos, baikal, cosmo, cyrus IMAP server supports calendars and contacts, davical, nextcloud/owncloud and bunch others if you would look for CalDAV servers

Git is a really great format for email archives, because it provides free duplication and compression via packfiles, although I think it is a tilting at windmills to try to map specific commits to emails. I explained my git/imap workflow in another comment here.

However, git is perfect for calendars. It is used as a backing store for these caldav servers:

https://github.com/jelmer/xandikos

Self-hosted xandikos in my case, but any CalDAV/CardDAV server works (Nextcloud, ...)

After looking for a long time and trying out different stuff, I ended up with xandikos. I use it together with thunderbird and davx5/davdroid, and have found that it works slightly better than radicale. I run it behind caddy in a podman container, but i think it's also possible to just apt install it.

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.