This is a good point. I assumed here that FS advocates will be basically opposed to a technology that serves to incorporate their code into software that does not provide the fundamental freedoms to end users, more than those who license their work permissively. But yes you could imagine an FS advocate who is quite happy to use the tech themselves and churn out code with GPL attached.

Fossil has a lot of features and config knobs.

If you are feeling ambitious and want to go “serverless”, try out DecSync and a compatible android app for contact sync. This represents all your contacts as files on disk in a way that avoids conflicts, and you can use SyncThing to keep your devices in sync 100% peer to peer. Unfortunately on your desktop you’ll probably have to use something like radicale on localhost and the plugin to convert it into CardDAV for your regular email client to understand.