Quad9 does also offer DoT and DoH.

PairDrop is a fork of SnapDrop, which at one point had more features and active development. Don’t know, how it is nowadays though.

I can recommend you ICSx5 for ical subscription and DAVx5 for calendar synchronisation.

Yet I would agree, that a good calendar app should ship with these features.

The German technology blog Kuketz has a comprehensive overview and comparison of all major messenger services.