It has no internet connection.
1- Can I allow content sharing (casting for instance)? Would it even work without giving access to the internet to the TV?
2- Does it matter which apps allow to modify system setting? There’s a huge list, here’s a snippet:
I don’t think it’s possible to share it unless you’re on the same network. Personally, I pushed NextDNS via ADB, along with hagezi ultimate and peerflyst’s smart tv blocklists. You can get a pretty decent result that way.
ELI5? 
Is that for question #2? I’m not sure how blocklist relates to the question?
Thanks for clarification 
That wasn’t the question. You’re just blocking traffic and making the device ‘stupid’
Ah okok, I prefer not connecting the device to the internet at all and connect something else where it’s easier to have more control on it like a PC.
So #1 is answered, #2 still remains.
I’d disable all but you will never have a guarantee that the thing won’t turn back on after an update or alike. 
It won’t break anything if I disable all?
Would be nice if we could flash a custom os on them.
