How to sync contacts between a GNU Linux desktop and an Android smartphone?

I currently use Google Contacts to manage my contacts on Android and PC (web app). But I'm trying to get rid of Google for FOSS solutions (I already replaced Gmail and Google Agenda). The first step would be to export Google Contacts data and import the data into a FOSS Android contact management (CM) app (e.g. Fossify Contact, OpenContacts, ConnectYou). Then find a way to sync the Android CM app with a GNU Linux CM app (preferably GNU Emacs) and be able to populate either the Mobile or the Desktop app continuously. I'm a complete beotian and don't understand how contact sync works. I read that CardDAV protocol should be used (I didn't find alternatives), either with (e.g. radicale or xandikos) or without a server (e.g. DecSync CC + Syncthing). I would prefer to avoid using a server, unless it is hosted by my smartphone on Android. Ideally, the solution would use Syncthing, which I already use for synching computers through my smartphone. I wasn't able to find tutorials on how to sync a FOSS Android CM app (i.e. not Google Contacts) with a Linux CM app (preferably GNU Emacs). Or advice on which would be the best (easiest to set up) combo (FOSS Android CM + synchronizer/server)...