I have had positive experiences with both Radicale and Baikal. I am not sure about the Home Assistant integration, but they both use CalDAV, so I would be surprised if there wasn’t a way of connecting them. iOS has native support for CalDAV, but Android needs the davx5 app (free on F-Droid).

It was in the news a couple of weeks ago. Here is an article: https://9to5google.com/2025/07/26/samsung-galaxy-one-ui-8-bootloader-unlock/

This is where I thought it was going as well.