I have entered a rabbits warren of attempting to setup a control panel for HA and also family planning. What I have found is there is no real solution for a local privacy centric solution to this that can replace say Google calendar.
There are a few options but all are fundamentally caldav and this means they integrate into HA OK but things like creating entries from HA becomes difficult
- NextCloud - is a sprawling mess and a massive overhead to run a calendar
- Baïkal - appears to be abandonware
- Radicale - still based on sabre which is abandonware (Development of sabre/dav and related packages are indefinitely on hold - sabre/dav)
Not really a HA thing but I see this as something for Open Home Foundation that would work with HA.
Something that works with Android and iOS and HA / web can’t be that difficult to implement but would be better than caldav which is a horrible protocol.