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.[1] https://radicale.org/v3.html
[2] https://www.davx5.com/