Recently had to buy a new TV due to a lightning strike (surge protector did nothing) and I never let my new "smart" TV communicate with the internet and it becomes a "dumb" TV and I feel pretty good about that.
The annoying thing is that even without it connected to the internet, it's still slowed down by the OS. I did the same and it's still not as responsive to things like switching inputs as the projector I was using before it.
Note I have my router redirecting all requests to port 53 (DNS) to the pi hole for those things that don't obey the DHCP DNS. Not sure if Roku is one of those offenders, but I haven't seen a Roku ad in years.