First thing you need to understand is that the smooth end-to-end encryption works only tuta-to-tuta or proton-to-proton, so in rare cases. Encryption at rest, which is what tuta-to-proton, gmail-to-tuta etc. can do, is something that a lot of other email providers do too.
I’m currently in the process of moving from Proton to Tuta, because despite several years of promises, the Android client for Proton still doesn’t do non-google push notifications. Also because if you just need email with your own domain, Tuta is much more price-friendly. (The tier also includes unlimited calendars and event invites, which I haven’t tried.) If you also want VPN and encrypted storage, the balance tips.
I don’t use the calendar from either, so can’t talk for their properties. I prefer seamless calendar integration for wrist gadget integration and such, so using NextCloud Calendar + DavX. For smooth integration with encryption, could also look into Etesync. I think you’ll be able to share an ics attachment from either of those through your normal calendar.
Germany is a 14-eyes-country, but since I’m just privacy conscious and my threat model doesn’t include international-coordination-level actors (barely state level, am in the EU but not German, so eh, far enough), it doesn’t matter that much to me. Proton also has to obey court rulings.