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 But that means that some flatpaks probably use outdated and unsecure dependencies.
Whats your opinion on that matter?
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.