From various places online, you can buy "scaler" boards that take HDMI/VGA/etc. to whatever interface the panel uses (LVDS is common), i.e. they comprise essentially all the electronics inside a standard non-smart monitor. They are intended for laptop display conversions but can drive much bigger displays too, as long as the interface is the same. That might be an alternative to buying a much more expensive "dumb TV", i.e. buy a smart TV and "dumb it down" yourself!
For example, this particular one is configured for an iPad's display but the sellers usually have other configurations available:
https://www.aliexpress.com/item/4K-eDP-HDMI-LVDS-Controller-...
Generally most modern TVs are gonna have a power supply, a main logic board, a display driver board and maybe another control board for stuff like backlights or sound. The logic board is basically gonna be an embedded mini computer running a full embedded OS that orchestrates the UI/smart features, decoding the inputs and generating the control signals for the panel driver. This will all be highly integrated on a single board, probably with almost all of the functionality in one or two main chips. Point being, you can't just yank out the smart stuff and still have a functioning TV. It'd be a bit like trying to remove a feature from a computer motherboard.
The ideal way to go about it would be to write your own firmware for the chips, but that's a practical impossibility because almost always there's no data publicly available on the chips or the other parts like the panel. You might also be able to get something working if the actual panel uses a standard interface, but it'll be a ton of work.
There aren't that many teardowns of televisions online that I can find, but this article has some to give you an idea.
https://www.techrepublic.com/pictures/cracking-open-the-55-s...
Now you're getting me paranoid.
Might be worth finding the network chips and disconnecting or breaking them off.