[easyKL]

Another route is to go serverless and use DecSync. Thunderbird (or Evolution) will sync with the Android app (F-Droid available) and your Cal, your Cards, and tasks, and even RSS will be replicated on the other peers.

https://github.com/39aldo39/DecSync

[FloatArtifact]

Interesting. I don't see any iOS support, and unfortunately it doesn't seem to be maintained.