Sync all contacts in a single vCard file rather than manually exporting it

[Anubis773]

Checklist

• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the request will be dismissed otherwise.
• This issue contains only one feature request.
• I have read and understood the contribution guidelines.

Feature description

I would like to have one single and local file with all my contacts, so that the app updates it automatically, and so that other apps can use this file as well.
The app should also look for changes in the chosen file and update the displayed contact list (this is why automatic backups don't work for that).

Why do you want this feature?

A counter-argument to this feature would be that Android already has its own local contact system. However, I would like to have access to my contacts on other devices (and I want them to be synced without having to manually import/export), which my phone only allows through an online account (such as Google).
Since I want my contacts to stay on my own devices, I cannot sync my contact this way. So the only solution I see is to have a vCard file synced between my devices using a software called Syncthing.
Unfortunately I haven't found any Android app that just lets me have a single file with all my contacts, and this is why I'm requesting this feature.

Additional information

No response