As far as I know, CasaOS (same as Cockpit) is installed on top of a default OS install, so you could always access the OS directly to install/configure things outside of it, if the need arises.

I would not say you would be held back by it, if it does what you need. And for what I can see online, you can install any docker container even if it’s not on the default catalog of CasaOS, or access the OS.

If you want to grow your knowledge of how things work, or how to deploy services without CasaOS, you can always do so in parallel of using CasaOS, so I don’t see where the issue could be.

I’m using Debian, with Docker and running Jellyfin, Nextcloud, Navidrome and Wireguard on Containers on my old laptop. So that would be my suggestion.

You could install CasaOS and/or Portainer, on top of Debian if you want an easier way to manage your server and containers.

If you are not behind a CGNAT, it should be as easy as opening the necessary ports.

I have a reverse proxy running in ports 80, 443 and can safely access Jellyfin on a subdomain without issues from outside my LAN.