I've never had any issues with Radicale, which is dead simple and lightweight. If you end up with Android again, DAVx5 has also never given me any trouble, and it also allows calendars to be cached offline. I'm not sure how you're having compatibility issues as I would think CalDAV is a standard protocol?
If you're concerned about dependencies and security, why not use Docker or Podman? It makes most of self-hosting in general much simpler, and it's much easier to secure since it's containerized. With containers, even if a hacker somehow hacks your CalDAV server, they can only access the minimal resources that you've given the container. I use this repo for Radicale on Docker.
Check compatibility for all your programs before you move. Most Linux programs work on Windows but not vice versa. If you're not in a rush, try switching to programs that have a Linux equivalent before you move so that you'll have less of a culture shock. If you need any killer apps that don't have a Linux equivalent you're going to have to make your peace with that ahead of time, otherwise you're just going to end up switching back.
KDE is a good choice, and Kubuntu should serve you fine; if you end up going with Kubuntu, I would recommend sticking with it for at least half a year or so before considering switching to something else, as that will give you time to really understand what you like and don't like about how Kubuntu and KDE work.