I’ve no idea on the relockable bootloader support with self signed keys. Though I do remember reading at some point from some lad ranting about why it’s a bad idea to relock, and seemed to make sense, so I didn’t look further into the matter. The post was on the… ehm… other website.
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.