A simple PC, you can also use it to browse, download stuff, Steam stream etc. The possibilities are endless

I’ve always loved my Apple TVs.

I blocked my TV from using WiFi, but there have been times I have need to unblock it (like using AirPlay or Google Cast), so it gets updates occasionally. I'm open to throwing my TV straight to the dump and buying another TV to avoid this at all costs.

Is nVidia Shield an alternative?

That's what I've been using for years now. It's great.

Personally looking at dumb panels and building OSMC machines to run them.

I run an LG OLED TV (disconnected from network), AppleTV, and my own media server. I haven’t seen an ad in my TV for years.

Nothing, all TVs are crap.

The best options are usually buying large "monitors" or digital signage. However these both tend to be more expensive than a similar TV. Monitors also often lack a remote which may be valuable for a TV and digital signage may have less input ports than you may want.

I would love if a major manufacturer made a TV that just displayed what signals I put into it.

Right now the best option still seems to buy a Roku TV and never connect it to the internet. But some features will be disabled. For example Miracast doesn't work for some explicitable reason until you connect it to the internet. (Then again it barely works anyways, so no major loss)

Get an A/V receiver, a computer monitor or dumb TV, and speakers. Then you can get a Roku streaming player and it cannot show you anything when you do not have its input selected on the receiver.

Even an inexpensive pair of bookshelf speakers placed on either side of the TV will sound better than built-in TV speakers. Add a center speaker and a subwoofer drastically improved sound.

Non-4k AV receivers are dirt cheap used.