Even if you do want to do casaOS, or linux- I'd still recommend putting proxmox as the base os.

Try each one and see which you like the most.

Depends on your objectives. If you care at all about learning the in-and-outs of how running those workloads actually function, go vanilla Linux (Ubuntu, Debian, Enterprise Linux, OpenSUSE, etc.). If you care less about that, but still want some control over the specifics, Proxmox is fine. CasaOS seems the most abstracted away from reality, although it looks like it would work just fine.

I generally recommend just using a regular Linux distribution. They'll generally be far more flexible and you'll learn more real skills. It's not like "control panels" don't exist either. Something like Cockpit could help you get started.

Proxmox. You can install any os on it to test it out.

Use Proxmox as your hypervisor then you can run as many and how many different types operating system you want on top of it all at the same time.

Unraid.

There is no better home server OS imo.

Why not both?

I run CasaOS inside Proxmox

Proxmox is a great solution if you want to quickly spin up discreet VMs.

If you just want a server to run docker containers then I'd recommend a Debian lite server with Cockpit webUI. It just streamlines so much and avoids the layer of abstraction you will find using something like Proxmox. Proxmox just seems a better fit for a larger server environment where you have loads of RAM and CPU cores to delegate out to each VM or LXC.

That has been my experience.