You could try this setup. Create a seperate network for TV with no internet (you might need a seperate rj45 connector on PI), then run microsocks proxy on pi (microsocks is super lightweight, and this will be using your other rj45 or wifi with internet), next install smarttube on TV and add your local proxy. You will need to create an exception in your firewall to allow connections to microsocks and block everything.
This way, nothing on your TV should have internet access other than any app you add that can accept a proxy.
If you don’t care about your TV spying on you, skip all this and just install smarttube on TV and use it.
Note: I just realized pi already has a wifi and an ethernet connection so you won’t need to buy another usb-to-rj45, so use one interface for no-internet-except-microsocks network, and use the other for normal one.
That’s a bit uncalled for, don’t you think?
I just used a mini pc to create a seperate network, then blocked internet for TV network. Then i setup microsocks and made an exception for that proxy and jellyfin. This way I can use smartube (it has a proxy option) for youtube and jellyfin for other media without logging in or providing internet access to anything on the TV.