I don’t think you need to configure your firewall. Firewalls are usualy used to block incomming connectings. Usualy a Firewall that blocks all incomming connections is already active on your modem/router. Adding exception to the modem/router Firewall usualy happen through port forwords.
Thanks for the helpful list. I had concerns in the past about flatpak, because as far as I know the dependencies are bundled into the flatpak and are not using the latest version of your distro. But that means that some flatpaks probably use outdated and unsecure dependencies.