DAVx5 basically acts as the connector between your server and your calendar/contacts/files apps. I would imagine that this could be built into an app, but there are a lot of ways that such apps can sync or operate locally. I’m guessing that it is just a little more specialized than most developers want to get.

Thanks for the Syncthing-Fork tip! For now the official version is working for me, but I’ll have to migrate myself soon.

From my understanding, OpenVPN provides the same secure remote access as Tailscale, by a slightly different method. You should be fine to use what you’ve already set up.