How about a TV with a slot for a Framework mainboard and maybe a drive bay or two?

Framework's USP is not making simple products, it's making repairable and customisable ones. I don't think there's anything to do there with a TV – there aren't bits you want to replace unless you make it a smart TV where you can replace "the smart" or "the TV" in it independently. You could make the I/O modular, but there's also no reason to not just include a ton of I/O on a TV because it's already huge and HDMI sockets are cheap.

The other issue here is that if you remove the crap/apps/smart/etc, you have to charge more, because TVs are typically subsidised by the post-purchase revenue flow that you can get from those features. Most people won't pay more for a TV that lacks features, that's a very niche market of HN commenters, and Framework have been explicit in wanting to move to less niche markets.

are TVs these days displaying ads over HDMI inputs or something?

what's stopping you from just getting any modern TV and not using the "smart" features?