Especially questionable choice by Proton not to opt for the self-hosted option. LiveKit offers an enterprise tier that even lets you set up your own mesh, so you are not dependent on their hosted infra.
10 karma · joined April 1, 2017
Have a look at the client (https://github.com/KinToday/kin-web-client) and the server (https://github.com/KinToday/kin-api-server) on GitHub. It's definitely inspired by Sunrise, but might be a suitable replacement for Google Calendar.
Even better: Contributors get the hosted version of Kin (2€/month; 20€/year) for free.