I think generally people use imdl in scripts nowadays, but I don’t know if there’s any GUIs or anything.

I use a 48TB ZFS RAIDZ2 pool to maintain data integrity locally and keep rolling ZFS snapshots with sanoid so that data recovery to any point within the last month is possible. Then I use borgmatic (borg) to sync the important data (~1TB) to a Servarica VPS (Polar Bear plan, which works out to be cheaper than Backblaze B2 costs for my purposes). The Servarica server really sucks in terms of CPU, and it’s quite sluggish, but it’s enough for daily backups. I also self-host healthchecks.io on a free Fly.io VPS thing (not sure if they offer this anymore) to make sure the backups are actually happening successfully, and hosting that on a third-party VPS means that it’s not going to fail at the same time my server does. Then I use Uptime Kuma to make sure everything is consistently alive (especially the healthchecks.io server, which in turn verifies that Uptime Kuma stays alive). I also run the same borg configuration to back up to a plain non-redundant disk locally.

The downside of this setup is that I’m only truly backing up a fraction of my pool, but most of my pool is stuff that I can redownload and set up again in the event of e.g. a house fire. I also run a daily script to dump a lot of metadata about my systems and pool, like directory listings of my media folders and installed programs/etc, which means that even if the data might be lost, I have a map of what I need to grab again.

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.