I have a sony as well, and you can use it as dumb TV, by not agreeing to their privacy policy. If you want to connect it to wifi afterwards, it points you to the privacy policy again, so it seems like the TV follows that. But I’ve also purchased it 4 years ago, so not sure what the status is now.

So far, I’ve seen no ads. I mostly use it mostly via HDMI and sometimes for watching freely available TV via antenna.

Have you looked at what sony has to offer? They latest xperia had a headphone jack, sd card and I believe lineageOS support.

i installed lineageOS on an xperia 5 ii, so older model, and seems to be going strong. got latest update 1 week ago.