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.