Yet again: It does not need a (new) calendar app, it only needs an app for syncing, Basically you can take DavDroid, fork it, add your e2e crypto and that’s it!
This is what etesync did, and their model looks solid.
Other platforms sometimes also have e.g. calendar apps (GNOME) or other software, which may get interoperable with an addon (Thunderbird e.g.).