Nice post, the discussion on Hacker News was also interesting. I recently came back to Proton for emails only. In the past I had issues with Proton Drive and emails because we can't use our own software and sharing with the outside world was not really user-friendly. For Proton Drive I ended up using my own SSD shared in a Samba share in my local network or using Nextcloud. Have you tried the calendar in Nextcloud? I use it to sync across devices. There is Qloud which provides such services with a free tier up to 5GB of storage I think.