OSMC is already shipped with a firewall: iptables. All it needs is a wee bit of configuration and, as you point out:[quote]
Configuring a firewall is not actually that difficult for most devices
But realistically, configuring a firewall to meet users’ unique needs is likely to be an unwanted complication for the majority of people, and a significant support burden for us here on the forums. And remember that firewalld cannot use an X-based interface, since OSMC doesn’t use X, so you’ll need to use the command line, plus AFAIK it doesn’t talk to connman.