As OSMC connects to the interne (though not set up for remote/external WAN access), is it worth installing a firewall (ufw)? And would I need to open any ports? (I only have youtube and iWWWplayer apps installed).
Generally, there seems differing opinion. Some say if a device is behind a nat router with built in firewall, that’s enough. Others say, having another layer of protection is necessary or a least increases security.
A device behind a NAT router doesn’t need a firewall unless you want to protect that device from your other LAN devices. But as the services on OSMC (SSH Server and HTTP Server) would be needed for access from your LAN restricting them may be questionable.
The only time you would need a firewall behind a NAT router (other than what I mentioned above) is when you use a VPN that gives you a public IP directly on that device.