Thanks for the nudge, I wrote to the EU registry.

On mobile I use Etar calendar app from f-droid, and DavX to synchronise calendar data. I use syncthing, but not for calendar sync, since the caldav protocol is designed for exactly that. But on the back end, if you want to host your own calendar, I recommend Radicale. It stores its data in simple text files, and works well with syncthing. Radicale.org

It’s not exactly DIY or RISC, but you might be interested in looking into purism. https://puri.sm

They also try to make or use open source hardware