Own your calendar and contacts with OpenBSD, Baïkal, and FOSS Android
An interesting alternative is Radicale [1].
Both are very lightweight.
I mainly chose Radicale over Baïkal because it is written in Python.
On the client side DAVx⁵ [2] do a good job.<p>[1] <a href="https://radicale.org/v3.html" rel="nofollow">https://radicale.org/v3.html</a>
[2] <a href="https://www.davx5.com/" rel="nofollow">https://www.davx5.com/</a>