Sure, you can use a VPN if you want to spin up the instance and connect to it without having a domain. You can always open the instance of Nextcloud to the internet later, when you buy a domain.
Get a cheap .XYZ domain if you just want to experiment with spinning up a reverse proxy.
As others are saying, OSMC might work. Most difficult part is making it so that the TV turns on when you turn on the computer since ARC isn’t a thing for most computers.
I ended up giving up on OSMC and bought an Apple TV since nothing else got the “wife approval” factor. It’s better than Google getting my data, has a Plex client, and let’s me stream my Steam library.