I have no experience with inviting others to calendar events since I only use this setup for my personal calendar - sorry

https://www.duden.de/sprachwissen/sprachratgeber/anscheinend-oder-scheinbar

Mir war das selbst lange Zeit nicht bewusst, aber seit ich es weiß, fällt es mir jedes Mal auf…

Idk about E2EE, but I’m using self-hosted Nextcloud. Using CalDAV, you can integrate virtually any app. On Android I’m using DAVx5 and Fossify Calendar.

The advantage of Nextcloud is that it provides a lot of things in one package such as contacts, tasks, notes, files, etc. If you don’t need those, it might be better to look at some of the other suggestions that only provide calendars.

Anscheinend*

Tut mir Leid, ich konnte nicht widerstehen…