Situation: On Android contacts are very easy to manage. On GrapheneOS they are even very secure with Contact scopes and in general Permission.
On Linux/Desktop this integration highly depends on your system, there is no Contact permission (even though I think Akonadi and the Gnome equivalent could do it).
So I backup .vcf files to a folder, with a script I delete duplicate entries. I would like to sync Thunderbird with that .vcf file, so that I can have all my mail adresses here.
I think there is a buggy addon for that, but it doesnt work great.
Having a central contact storage is so essential for this. Having this for Thunderbird-K9mail-Sync would of course also be an option, whatever that may be.